Toughest as per firepower? Toughest as per shooting down? I assume you mean toughest to shoot down...
P-38 - Yeah, it's a bit fragile, but flown right it's a downright tough bird to shoot down
P-47 - Seems fragile in game by some, but I've soaked up many, many 50 cal, 20mm, even a couple 30mm cannon rounds and still got home.
F6F-5 - It pack's a punch while having the ability to take a punch. I've survived 30mm hits to the nose before, and countless hits to the wings and tail.