The T-34 tank

A diesel HVSS Firefly Sherman would be as good as any T-34/85 in overall performance. One only needs to see the Israeli upgraded Shermans to see the amount of modification the chassis could do.

The main problem the Americans had was their combat inexperience compared to the other combatants, their conservative gun design practices (preferring low chamber pressure thus avoiding heavy propellant charges in ammunition to prevent faster erosion, but sacrificing velocity), the system and procedures for introducing new equipment and the monumental ineptitude of Lt. Gen. Lesley McNair, chief of the Army Ground Forces, who deliberately subordinated the tanks' anti-armor capability to that of his favored tank destroyers. Just compare the US M1 series of 76mm guns to the Brit 17-pdr of the same caliber. I believe a 76mm gun with, say, 8lbs of propellant in the ammo (instead of 3lbs 6oz)would give it about 48.000psi of chamber pressure (instead of about 38.000psi) and a much higher muzzle velocity, although at the expense of some barrel life, thus making it capable of dealing with Tigers and Panthers with a greater prospect of success. And a good move would be to arm the M4A3E8 with the 90mm gun too (that was attempted by fitting a Pershing turret on a Sherman). The T23 type turret (76mm models) was certainly capable of accepting such a large weapon, qv the 105mm French gun on the M51 Isherman, as montyp165 wrote.