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.
Let me fix something:
M4A3E8, not M3E8.
And one thing- the Panther's 75 mm was simply long.
This is rediculously long:

88 mm cal, 71 calibers long (6.25 meters).
To reach ludicrous length, we need the 128 mm/L55 (7.04 meters)

I knew about the Pershing, just that they weren't exactly as common as the Sherman was. Although I could sworn that the Pershing was the eqivilant of the Tiger
Thanks for the correction on the Sherman.
I think the M26 Pershing was the best WW II tank, even though it came to the front lines in Feb 1945.
Those things weren't heavy enough to be a pain in the @$$ to be moved around the battlefield (41.9 tons, vs ~45t Panther, ~57t Tiger I, ~70t Tiger II or 61.4t Abrams), they killed Panthers (and a few Tigers and King Tigers with a little luck too), and in the Korean War they scored half of all T-34/85 kills, showing that they're better not only than the German, but Soviet tanks too.

Pershing behind an unfinished German E100 prototype
BTW- to get the thread a bit back on topic:
The P-1000 Land Cruiser
Ratte, armed with 2 280 mm naval guns, a 'secondary' 128 mm and a bunch of AA guns, was meant to weigh 1000 tons. Too bad it never made it to production, Hitler would have killed himself as soon as he'd see the refueling bill, ending the war on Day 1 the P-1000 was deployed. The monster was to be powered by either 2 U-Boat engines or by 8 torpedo boat engines, giving it up to 17 000 HP, and a road speed of 40 km/h.

(note the 128 mm is in the hull, because the turret mounted guns are too high for close range fighting)

(dimmentions in milimeters)

(2 twin 37 mm Flak turrets on the back)