They were tanks, but they were no way unstoppable. The english demostrated it three times at Crecy, Poiters and Agincourt
Turkish and Muslim missle cavalry slaughtered Crusader Armies.
And relatively small numbers of heavily armored knights often demolished lighter Muslim cavalry and infantry during the crusades as well. Horse archers were a constant menace throughout history until the development of better firearms, but the tactics to defeat them remained largely the same since Alexander's days. Maneuver them into a corner than bring down the hammer (often heavy cavalry) to crush them.
Of course that's easier said than done, but arrows usually did not pierce the heaviest armored knights and they could withstand the volley's until the horse archers ran out or until they were able to push them into tight terrain or up against an obstacle. Sometimes the obstacle was geographic, sometimes it was infantry forces playing anvil.
The heavy cavalry was countered by the Welsh longbowman, but those tactics were not employed throughout Europe. The strength and skill to effectively wield that weapon took years of training and thus it remained a trump card of the English/British rather than revolutionizing european warfare. Firearms, rather than immediately obsoleting the knight, saw the rapid improvement of cavalry armor. Your standard musket could not penetrate the better cavalry armors (unless up close) until well into the 17th century. (might be off by a century) Cavalry armors could also counter many crossbows as well.
The swiss pike formations were another matter. Frontal charged against a well disciplined pike formation was suicide. But such idiocy had been largely suicide since the days of the Greek phalanx. The smart army flanks. And even a well disciplined, veteran unit hit from the side or behind by a heavy shock charge is gonna buckle and likely break. It was down to manuever and properly softening up the pike formation (arrows were a start, cannon was better...at least once the cannons became more effective) before hitting them.
The heavy shock charge could be countered. And there certainly have been several examples of it in history. There have also been other examples in history of the shock charge demolishing those sames sorts of tactics and armies. Heavy armor of course receded and the shock charge diminished. But it did not disappear. It made frequent appearances in the Napoleonic wars and could even be seen in the American civil war, the eastern front of WW1, and even bits of WW2.
The shock charge was one of the most effective tools of warfare for almost a thousand years after the development of the stirrup.
That's my nomination. (heavy cavalry)