Among the pure fighters I'd give the title to the F6F, with the FM2 close second.
The A20 is a special category being a bomber that is flown like a fighter-bomber and is incredibly tough. I killed one using only the .303 guns of the mossie and it took about 2000 bullets to do so - I was unable to put all 2000 on the same spot because the A20 wouldn't sit still as I was aiming. I was really hosing it with a continuous stream till finally something vital fell off. Even with the mossie quad 20mm they often survive a good volley.
The mosquito is more durable than most single engine fighters in the game, but it is no where near the A20 scale. It has two glaring vulnerabilities - one is that it tends to catch fire more often than most planes. I attribute that to wing tanks that cover a larger area than fusellage tanks and thus are more likely to get hit. The AH Mossie have 4 wing tanks and one aux tank. The other is a high rate of pilot wounds. Real life mossies were considered very durable by their pilots, often surviving flak hits and flying 4 hours on one engine to land at home.
