P-39 as a plane that could impact the two more deficient plane sets--Eastern Front and Pacific.
Ki-43 would be my first choice, with one caveat...Its use in the MA could run the risk of being more cult-like, but for the SEA, it would be huge. The rest of the Japanese aircraft mentioned were comparitively much less significant. The fact that the IJA flew the Ki-43 for the entire war when considered with the number built, IMHO means this should be the next Japanese plane added to the set. I suppose that if some of the different versions were included, it could even be considered competitive in the MA.
B-25 gets my second vote based on numbers produced, significance of its contribution to the war in almost ALL theaters, and that it also, like the Ki-43 was operational for practically the entire war, although in different versions. B-25 would also be competitive in the MA. My second plane would be the Betty, except for its value in the MA. For SEA, it again would be huge. But, given the need in my opinion for some utility in the MA's, I'd have to say the B-25 tips the scales in its own favor.