Sweet choice Owen, mental thing.
If 'best' refers to 'most mental' I might go for the 38cm RW6 anti-submarine rocket mortar on the Sturmmorser Tiger. (
very rare footage of it firing here:
MENDEN PICTURES)
Realistically I may just choose the 75mm KWK42 L70 as fitted in the Panther, as I believe it outshot the L56 88 and seems to be a far more compact and usable package than other meaty 'classics' from ww2 like the 17pdr which were only mounted in vehicles with some difficulty and associated problems.
The head-scratching continues... those 90mm's fitted to American TD';s seem to have been pretty effective too. And there's always that twin 25pdr equipped Sentinel IV (17pdr emulator) project....