Erm, let it be said that McClellan was perhaps the greatest army administrator that the United States has ever had. As a Chief of Staff, for example, he was unrivaled. The Army of the Potomac was superbly supplied, organized, etc., and the men loved him dearly.
That, of course, does not make up for his on-the-field caution and such.
The Civil War was also a showcase for excellent American military leadership on both sides of the field; men like Hancock, Gordon, Thomas, Sherman, Sheridan, and Forrest were quite up to par with Europe's varsity like Constantin von Alvensleben, Skobelev, and the elder Moltke. America had plenty of solid professional officers as well, men like Grant or Meade. There were plenty of poor military leaders in European contemporary armies as well, and not just the oft-decried Bazaine and Benedek - the Prussians had their fair share of blunderers, such as Karl von Steinmetz and Friedrich von Wrangel.
The United States also produced superlative naval leaders such as Porter and Farragut; as far as individual leadership went, their only contemporary in Europe was the Austrian Wilhelm von Tegetthoff.
I'd say that Sherman was perhaps the greatest army commander, on either side, during the war. He had his fair share of off-the-field problems, but [if you get to know him] he was actually quite an interesting guy. He was quite philosophical, and his campaign in Georgia/the South was certainly better than Grant's in Virginia.
If we count commanders
in general, JB Hood as a brigadier/divisional commander was one of the greatest. On the division level, his aggressive nature and tactical flairs made all the difference in several engagements; his Texas Brigade was Lee's elite shock troops. The only problem was, as an army commander, he ordered the army around like one giant division, rather than many separate entities - I don't think he realized that army command, rather than brigade/division command, required much more finesse.
As for naval commanders, Buchanan on the Confederates stands out simply because of the lack of resources he had; he was still able to organize, field, and command the Confederate navy, despite the odds stacked against him.
I like your comparison to Europe too.
[I'll stop now before I start ranting....lol, once I start I can't stop with this history]