For just plain vicious prototypes the US T 29 heavy tank is right up there

T29 Super Heavy Tank

The hull with slope is 8" thick in front while the turret front is 8 to 11" thick with 7" sides and a 5" thick rear.
The main gun is the 105mm T5E1 that could penetrate about 6" of armor with solid shot at 1000 yds, 5" at 2000 yds, and with HVAP could drill through almost double that at either range.
It could manage 22 mph and weighed about 70 tons.