Looking at this purely from the perspective of which airplane would actually get serious use in the arena, I'd have to vote for:
B-25
Yak-3
A-26
All the other planes are cool and all, and have their various appeal, but would anybody (except for the few who love each plane) ever fly them in the arena?
Actually, I'd vote for the Yak-3 (as it would be easier to modify the Yak-9 rather than start from scratch) AND one of the medium buffs. To choose between the 25 and 26 is tough, but I'd go with the later versions of the 26 as a perk buff - if only to give something to spend buff perks on.
The Do-335 would definitely get my vote though.