In the early stages of the war, the USSR had a lot of very different tanks, some of which were crap, some of which were the best tanks in the world at that time. I don't think any other country had experimented as much with different types of tanks as the USSR had. In production, of course they quickly settled on the tanks that were clearly making the difference: the KV tanks and the T-34.

But even then, the KV tanks were only a small part of their arsenal; the Russians had settled on three different categories of tanks: mobile tanks (their very fast BT tanks using Christie suspension), medium tanks, and heavy tanks. The latter category included the KV-1 and KV-2 tanks, and that was the smallest and most expensive group, but also the group that would always be at the center of any attack or defense.

The reason the T-34 was such an important tank, is that it combined aspects of all three categories; it was intended as a medium tank, but it had Christie suspension like the BT tanks, making it faster than most other medium tanks. And due to its sloping armour, it was almost as tough as a heavy tank (against tanks at least; it was vulnerable from the air). This combination made it the precursor of the Main Battle Tank.