A Sherman with the high-velocity 76mm gun (they did indeed upgrade them, though it was not always consistant; some fought to the end of the war with their original guns) was a capable enough tank for most purposes; it would not take a Tiger, but neither would the T-34. The Panther could penetrate armor better, but the Panther had the ridiculously long-barreled 75mm/L70 with the muzzle velocity to beat almost anything. Still, one on one, a Sherman/76 would have had a fighting chance against a Panther and better-than-even odds against the Panzer IV. And the Sherman was subjected to continous upgrades; the M3E8 version with improved frontal armor could shrug off an 88mm/L70 hit at a thousand yards, something no version of the T-34 that ever went into combat could claim.
To say nothing of the Brit variations on the Sherman mounting the superb 17-pounder antitank gun, which could take a Tiger. The VC Firefly was probably the best medium tank the Western Allies produced. If there had ever been an M3E8 with the 17-pounder then it would have been the best Allied medium tank period.