E8 really wouldn't be uber, even with the 76mm.
Remember, only difference between the M4A3 and the M4A3E8 is the suspension system. Same armor. Slightly more turrent armor with the 76mm ring. Just a bit faster.
The 76mm ballistics (without HVAP) are not as good as the Pak 48 (magnification limitations would be needed for it and the T-34/85)
T-34/85 is till the fastest, has slightly less hull armor but sloping makes it much more effective then the e8's and a thicker turrent cast. Best gun in the HE department due to shell size.
Perks...Ya I'd say M4A3E2-76 (Jumbo). over 200 delivered after the Normandy landings, usually coloumn leaders when available. The only Sherman US crews liked

6 inch cast turrent ring, nearly twice as much as the King Tiger's 80mm's. (which makes up for the sloping of the KT's turrent.) and almost as thick in the hull (again with out sloping though) Little armor in the rear. Was only slightly slower then the M4A1 Shermans, nice trade off.
Panther G, easily the king in most areas.
Soviet and British tanks all depend on what ya want. You can go with BFG types, such as the Su and Ju (Su-152's and Ju-152s

)...generally fast but less armor then the IS series.
VC Firefly would be as deadly as the Panther G, but without the armor.
- Jig