The question itself is meaningless; whoever is insisting it was the T-34 sounds ill-prepared to discuss the subject intelligently. First of all, you need to frame the discussion. What do you mean by "best"? Any tank is a combination of several factors, all compromised to ensure maximum potential in each area - speed, armour protection, ground pressure/mobility, crew survivability, fuel consumption, main armament capability vs soft targets, main armament armour penetration ability, special ammunition, amount of ammo safely stowed on board, ease of maintenance, weight (important when crossing bridges, for example), size (important in narrow European villages), etc.
The T-34 and Sherman were aided by the fact they were produced in such staggering numbers. German AFV production was mainly armoured half tracks (a sizeable proportion of their inventory), and the number of "good" tanks after 1943 - ie tanks capable of going toe to two with enemy armour - was low.
The Russians beat the Germans with manpower, firepower, and an operational art that exceeded that of the Germans by 1944, who were never very good at broad or long term thinking (and when they were, they were hamstrung by having a leader who was political figurehead, head of state, and chief of the Army combined).
There was no single "best" tank of World War Two, nor could there be, unless your friend is prepared to give out his definition or frame the discussion better.