I prefer the 30mm mk103, any hit ensures a kill on all fighterplanes, maybe with the exception of one.
Its the only plane you get a full set of bombers with one HO, if you are slow and perfect positioned.
(Done it in the 190a8 with 30mm's also, but I normally only get one or two)
B26's seem to be hardest to kill of them all, and they bite back hardest too.