The M-26 is about as gnarly as it got for the US in WW2. The M-36 "Jackson" had the same gun, but it was a tank-destroyer - too thin-skinned for open combat - and open-topped as well.
There was a variant of the M4 that mounted a 105mm howitzer which would be great fun at knocking down hangars. As would the flame-thrower version issued to the USMC (though I suspect that would entail coding up fire-damage).
