At first I clicked "A-36", and then I realized that the P-51B doesn't get used much anyway, so why bother with an even earlier model. I also realized that there were only 5 Japanese fighters - A6M2, A6M5, N1k2, Ki-87, and Ki-61. Of those, only 3 are used with any regularity in the MA. So, I voted Ki-43, as I'm waiting to see if a pair of nose-mounted 20mike-mikes in an Oscar will bring any players to the table. A lot of Japanese aces got their kills in one, too. I'm hoping some day the Ki-44 Shoki will be available, along with the French D.520.
I thought about voting Pe-2, but anything it can do the IL-2 can do better; or any of the American bombers.