Personal favorite: FM2-While not many "aces" (5) flew it, they had a very respectable kill/loss ratio. The FM2's more powerful engine, lighter weight, larger tail control surfaces and increase in performance (climb, speed and turning ability) caught many Japanese pilots by surprise. There is even a case where a FM2 shot down a Zero while it was shooting up a Corsair during that aircrafts introduction to the war. Had the FM2 been introduced sooner, I believe it would have produced a considerable number of aces in the Pacific.
While outnumbered, slower, less manueverable than its antongonists, the whole Wildcat series seemed to get the job done. It's ability to take considerable damage and bring it's pilot home, operate in some of the worst conditions imaginable, operate from small carrier decks and dish out a good amount of damage, allowed the creation of excellent combat pilots and several aces who would go on to the the Corsair or Hellcat.