A number of factors should be considered in my opinion. Was the tank STRATEGICALLY influential on the war? Was it reliable in that numbers were available to be on the battlefield when needed? Could it fulfill the main role (in WWII tanks spent most of their time blowing stuff up with HE rounds) of a tank well? Was it available in numbers? The Tiger tank had a powerful gun but was built in small numbers and guzzled gas so greatly it could not stay in a moving battle for long. This tank represented a tactical winner but could not influence the war strategically.
The T-34 had a few flaws which took it out of this contest. A four man crew is a major design flaw for this tank and resulted in the destruction of many T-34's as the commander of the tank tried to perform double duty. The USSR needed to field tanks fast and Stalin called for a reduction in time and expense in the manufacture of this tank, resulting in a reduction of quality as well. These two factors, I believe, resulted in the T-34/76 receiving a poorer rating and forcing it into second place in this contest.
The Sherman was an extremely reliable tank, easy to service, easy to modify and showed great versatility (numerous other AFV's used the same chasis as did the M4.) Flaws in the tank were not difficult to alter and fix. It moved well, had good visibility, good firepower (remember what tanks spent most of their time doing in this war), good internal layout and could be armed with any number of guns (75mm, 76mm, 17-pdr and 90mm if necessary.) The M4 was used by every ally using armor in the European and Pacific theater and thus represented a tremendous strategic influence in the war. I don't think any tank can boast the credentials of the M4. (This contest should have included a few other tanks, btw.)