The Red Army of 1941 was, to put it bluntly, a catastrophe waiting to happen, and this was in large part due to its leadership. It has been fashionable for many people to argue that the effect of the Stalin-era purges has been vastly exaggerated. In civilian life, that may well be true. In the Soviet military, however, there is still virtual academic unanimity that the purges left leadership a hollowed-out husk. Many of the best officers were dead or imprisoned. Those that were left were usually terrified into submissiveness. Few outstanding officers remained in positions of high command. Many of those who were, were cronies of Stalin.
K. E. Voroshilov is probably the most egregious example of the latter. Voroshilov was not a complete idiot. He understood that the purges had gotten rid of better leaders than he was. He also understood when his own forces were not up to par. In his report on the 1936 maneuvers, he initially described the exercise (which British observer Giffard Martel characterized as two forces that "appeared just to bump into each other") in glowing term, probably to avoid appearing incompetent, but admitted to serious command and control problems later in the same document. Candor aside, however, his military ability can be charitably described as "limited". He was a member of Stalin's cavalry mafia, however, so he kept high leadership: member of the GKO, chief of a Strategic Direction, Defense Commissar, and so on. He was not up to the demands of modern warfare. He understood when subordinates lacked drive but possessed none of his own. His mismanagement of the Winter War was one of the epic military catastrophes of the twentieth century. He exhibited personal bravery on at least one occasion early in the Great Patriotic War during the defense of Leningrad, but at the same time was totally failing to organize his troops effectively. He was quickly replaced in that post by G. K. Zhukov, a much better choice.
S. K. Timoshenko, one of the other notable early Soviet commanders, was on balance more competent than Voroshilov. Timoshenko's generalship can be reasonably described as unsophisticated. The stereotype of Soviet soldiery repeatedly flailing away on the same axis of attack for little gain is an unfortunate one, but in Timoshenko's case it was reality. He was a general who could at least motivate his soldiers to fight, and fight hard. It was Timoshenko who kept the entire Western Strategic Direction from collapsing entirely during the ferocious battles for Smolensk and El'nia in the summer of 1941. In doing this, he expended a tremendous amount of manpower and materiel. He also probably saved the Soviet Union, by making it impossible for the Wehrmacht to continue east toward Moscow. He also led the counterattacks in the south in the winter and spring of 1941-42 that put so much pressure on the Germans...and which also led to the last great German encirclement of the war, Operation FREDERICUS, which annihilated several armies numbering a quarter million soldiers. Timoshenko undoubtedly lacked flexibility, and Stalin was probably correct in replacing him in 1942.
Another hard fighter of the early war was M. P. Kirponos, chief of the Southwestern Front, who defended Ukraine from Rundstedt and the German Army Group South. Much is often made of Kirponos' leadership in Ukraine, where the Nazi spearheads were badly stalled until help came from the north in the form of Guderian and Panzer Group 2. Had the rest of the Red Army held on like Kirponos and his troops, so the argument goes, the fascists might have been stopped long before Moscow. Of course, this ignores the fact that the Germans had little room to maneuver in the opening stages of the fighting in Ukraine, that Kleist's Panzer Group 1 was eventually able to slash through Soviet defenses too, and that Soviet forces suffered disaster in the Uman Pocket long before Guderian's panzers went south. Kirponos was unquestionably a fighter and was better at handling his armor in a counterattack than were many of his fellow Soviet officers early in the war, but both he and his instrument were not up to the task of fighting the Wehrmacht in 1941.
You'll notice a trend here. The most effective Soviet leaders early in the war were mostly the ones who were willing to spend their men's lives to kill more Germans. This was probably unavoidable, at least to some degree. The Red Army was not nearly as mobile as the Wehrmacht. The VVS could hurt the Germans but was totally incapable of protecting ground forces from the Luftwaffe. Soviet leaders were under supply pressure nearly as immense as that which faced the Germans, and transportation problems led to road marches that crippled many units before they even got to the front. Under such circumstances, generalship could not consist of formulating complex, balletic maneuvers to stupefy the opponent, to say the least. However, staff failures, and command failures, contributed to all of those problems.
The early war is littered with the careers of Soviet generals who simply lacked the ability to command effectively. D. G. Pavlov, commander of the Western Front, was generally incompetent, although he was simultaneously placed in an impossible position; his failures in the opening days of the war got him accused of sabotage and executed. He was later exonerated of the sabotage but will probably never be exonerated of the incompetence. Other officers managed to survive despite incompetence, like Voroshilov (moved upstairs) and Timoshenko (shifted to an advisory role). D. T. Kozlov's incompetent command of the amphibious assault on Kerch during the Crimean campaign in 1941-42 led to the destruction of virtually his entire force of two armies during the German TRAPPENJAGD counteroffensive; he continued in various posts in the Transcaucasus and Voronezh Fronts before being shuffled off to the Transbaikal where he couldn't do any more harm. I. E. Petrov also had a poor showing in the Crimea as the chief of the Coastal Army; he was more of a fighter than a manager, and his defense of Sevastopol was haphazard. Somehow he was given the responsibility for liberating the peninsula in 1944, which he also bungled; most of the Germans and Romanians got away.
Eventually, the Red Army developed to the point where it no longer had to expend soldiers' lives as an alternative to maneuver and materiel. Some generals were better at recognizing that fact than others. K. K. Rokossovskii was probably the finest Soviet operational commander of the war. He proved his mettle in virtually every situation imaginable: the swirling armored melees of Ukraine, the grim slaughter of Smolensk and Moscow and Stalingrad, the high-technology apocalyptic showdown at Kursk, the battles of annihilation in Belorussia, and the final march through Poland to Berlin. He rarely made a misstep (Belorussia in 1943 being possibly the only example) and demonstrated real genius at the operational level on both attack and defense.
Many of the other late-war Soviet marshals were true competent professionals. R. Ia. Malinovskii was a gifted armor commander who made his way up to Front command and masterminded the liberation of Hungary and Romania. (Interestingly, Malinovskii was one of the few Soviet commanders who had to interact with an allied force, namely the Romanian army after that country ended its association with the Axis in 1944. While the Romanians didn't exactly slot into the Soviet order of battle seamlessly, the two forces cooperated with each other reasonably well considering the circumstances.) I. K. Bagramian and F. I. Tolbukhin also demonstrated real skill in their assignments. Both of them were prone to commit missteps early in the war (Bagramian in his first field command at Sixteenth Army, Tolbukhin as a staff officer in the Crimea campaign in 1941-42), but both clearly improved as time went on.
Other Soviet officers of the late war lacked flair, but were broadly capable of doing their jobs. V. I. Chuikov gained historical immortality for his leadership of the stubborn defenders of Stalingrad in the dark days of 1942. As a commander in most situations, he was unspectacular, but frankly, leading the defenders of Stalingrad ought to be more than enough for anybody. G. F. Zakharov showed good aggressive spirit as an Army commander, but when elevated to Front command in 1944 the powers that be were displeased with his performance and he was demoted again. I. D. Cherniakhovskii showed general competence and some flashes of brilliance, especially during the Belorussian Strategic Offensive Operation, but the bloody operations to capture East Prussia in 1945 did not reflect particularly well on him (although they reflected even more poorly on Stalin and the GKO, who could easily have masked the province to maintain the drive on Berlin).
In sharp contrast to the Germans, the Soviet military leadership was also more willing, as a group, to take risks as time went on, a generally positive development. This did not mean that fear of Stalin and the NKVD was a thing of the past - far from it.
For example, during the Battle of Kursk, the commander of the Voronezh Front, N. F. Vatutin, became acutely aware that the German offensive was proceeding much more rapidly than he'd planned. While the Red Army's forces could contain the Germans at virtually any cost due to the vast reserve force assembled in the Steppe Front under I. S. Konev, Stalin and the GKO wanted that reserve deployed for counteroffensive operations only. Voronezh Front's objective was to defeat the German attack on its own resources. Within a few days of the beginning of the attack, Vatutin understood that this was no longer very likely.
One of the key German advantages was in their armored formations. The heavy tanks - Tiger, Panther, and the Panzerjäger Ferdinand - got the publicity, but many of the German formations relied on long-barreled Panzer IVs with high-quality optics. Whether the Nazis had Tigers or Panzer IVs, though, they outranged the Soviets with their increasingly obsolescent T-34/76s. Using the T-34s doctrinally, in a counterattack role, was suicidal. The T-34 was too large of a target and was too ineffective against German frontal armor for too long. German panzers picked them off at range like the armored equivalent of a sniper. (Later on, during the armored clash at Prokhorovka, P. A. Rotmistrov sardonically told the crews of Fifth Guards Tank Army to engage the Germans in close combat "and board them", as the only possible means of effectively coming to grips with the SS Tigers. Rotmistrov's plan did not work and his tank army was thoroughly wrecked.) One of Vatutin's high cards before the fight started was First Tank Army, under the outstanding armor officer M. E. Katukov, but with the German tanks so badly outranging the Soviet T-34s, First Tanks was increasingly looking like more of a joker than a face card. In response, Vatutin decided to deploy First Tanks in hull-down positions, dug in to the turrets. It would make them almost impossible for the German gunners to acquire and kill. It meant that the T-34s could not be used in a counterattack role, and this violation of doctrine meant dire things if it failed. Vatutin, however, was willing to risk it, even though it made him vulnerable to Stalin's wrath, because he knew that it only made him vulnerable to Stalin's wrath if it failed.
Vatutin was probably the Red Army's premiere risk-taker. Sometimes the risks didn't pay off. Vatutin was known for getting overaggressive, overextending, and biting off more than he could chew. His advance forces, including Group Popov, got annihilated during Manstein's "Rochade" counterattack in 1943. He again lost his spearheads to a German counteroffensive during the struggle for Kiev in the fall of the same year, when Hermann Balck's XLVIII Panzer Corps wrecked a Soviet tank corps and halted Vatutin's drive on Zhitomir. Risk-takers, however, were what the Red Army really needed, and contrasted very favorably with the unimaginative and destructive command style of many early war commanders. Vatutin had a real gift. which he demonstrated in the successful battles to reach the Dnepr River and in the Korsun-Shevchenkovskii Offensive Operation, and his death at the hands of Ukrainian partisans in 1944 robbed the Red Army of a general of genuine talent.
Some Soviet officers, however, did not grow beyond the disciplined but unimaginative stereotype of the early war years. G. K. Zhukov and I. S. Konev are probably the most famous Soviet marshals of the war in the rest of the world. They certainly had talent. Zhukov in particular was persistent to the point of stubbornness, possessed boldness and initiative and discipline, and, most of all, was aggressive. All of these are positive military traits, to a degree. That stubbornness, however, often led him down unfortunate roads. It took a total disaster for him to finally admit defeat and change his approach. Konev was in many ways broadly similar. Both men participated in the endless Rzhev campaign, which resulted in the deaths of hundreds of thousands of Soviet soldiers over the course of an entire year with virtually nothing to show for it. It's fair to say that Zhukov, at least, was obsessed with the place. He attacked an incredibly tough target unimaginatively and his men paid the price again, and again, and again. Zhukov and Konev both repeatedly showed that even in successful attacks, they were usually less efficient (i.e. they got more of their own men killed) than other Soviet operational commanders. The casualty rate Konev's men suffered in the offensive against Army Group North Ukraine in late summer 1944 was simply embarrassingly, only matched by Zhukov's horrifying mismanagement of the Berlin Strategic Offensive Operation in 1945.
Frankly, both Zhukov and Konev were butchers. They were the Red Army at its worst in the later years of the war. While many Soviet commanders fully understood operational art, and had developed effective tactics to counter the simultaneously-evolving German defensive tactics, others, with Zhukov and Konev chief among them, continued to bull ahead with relatively little concern for means to achieve the same result with fewer casualties. One of the most horrifying facts about the Great Patriotic War is that in almost every battle of the war, save perhaps the last few months of it, the Red Army suffered more casualties than the Wehrmacht - more men killed, more men wounded, more equipment lost or damaged, and so on, and so forth. The stereotype is that the Red Army could easily sustain those casualties. It could not. More than once during the war, the Soviet Union started to scrape the bottom of its manpower barrel, and conscripts from newly-liberated territory made the difference. By the end of the war, the Red Army was a truly powerful force of high quality and great quantity, led in the main by competent, seasoned professionals every bit the equal of their German rivals. But its great flaw remained that it took horrendous losses. Many of these were unavoidable, the result of a grim defensive campaign waged by a still-formidable foe. Many of them were absolutely avoidable. Officers like Rokossovskii did their best to avoid them; officers like Zhukov did not.
The best of the Soviet marshalate was not Zhukov. Instead, in my opinion, it was A. M. Vasilevskii, who spent much of the war as Chief of the General Staff. Vasilevskii was B. M. Shaposhnikov's protege, and showed his boss's attention to detail and planning in most of what he did. Stalin recognized his ability and from 1942 onward made him one of the "troubleshooters", Stavka's representatives sent to the front who could do anything from kibitzing and dispensing advice to actually assuming control of the battle. When the USSR launched its two grand offensives in the winter of 1942, Zhukov was sent to command one and Vasilevskii the other. Zhukov's attack, which was better-resourced, was Operation MARS, the climax of his obsession with the Rzhev salient, and it ended in bloody disaster due to command mismanagement. While Zhukov poured out his men's blood in the snow, Vasilevskii orchestrated the first great Soviet victory of the war by encircling and annihilating the German Sixth Army at Stalingrad. One could not imagine a better way to contrast the two. Vasilevskii was a more thoughtful soldier. He rarely shied away from difficult (bloody) fighting, but did what he could to mitigate it and do it more cheaply. Furthermore, his staff work inaugurated a new era of remarkable effectiveness in Soviet organization; one can say that without Vasilevskii, Soviet operational art probably wouldn't even have existed. He was an excellent officer and one of the finest generals in Russian history.