I think the best version of the P-39 was the Q-10 (or was it Q-20?). It had the two extra .50 gun pods in the wings omitted thus improving the roll rate and top speed. Armament consisted of 2 .50s in the nose and the 37mm.
The P-39 Q-5, Q-10, Q-20, and Q-15 were the largest production runs in the Q series. (Which in turn was the highest production run of any version)
If we got the P-39 in Aces High, I'd like to see the 3 major revisions modeled. Or maybe just 2 versions with an optional loadout for gunpods.
Actually, ANY version would be nice.