The would-have-been best, if…
A good example of a supposedly “perfect” fighter that’s cited by many historians as being the greatest such WW II aircraft, is the German twin-jet-powered Messerschmitt Me 262. It was more than 100mph faster and had a more powerful fighter-to-fighter armament (four centerline, aimed 30mm cannon) than any other WW II fighter. But it had three serious combat problems that clearly keep it out of the “best” category.
From the beginning, the Me 262’s engines were problematic. They were extremely sensitive to throttle movement and could easily blow out their combustion chambers; they were also exceedingly hard to restart in flight. Even if the engines had been perfect, however, the airplane had no propeller drag that would allow speed control by changing power; all WW II fighter pilots used this feature when combat maneuvering. The Me 262 did not have the speed-control capability required for two-plane fighter combat and formation flying.
Even with these problems, however, it would have been among the best if enough had been built and put into combat to make any real contribution to the German war effort. Thanks to Hitler’s many incredibly shortsighted decisions, only 160 hurriedly flight-tested Me 262s were available for combat operations about a year before the War ended. I cannot even consider it a contender. Was the Me-262 a good fighter? Absolutely! Was it good in the ground-attack role? Probably. Did it make a major contribution? Of course not.
Another would-be contender might be the Curtiss P-40 Warhawk; 13,733 were delivered between 1940 and 1944. Although it had some well-known limitations, it was on the USAAC’s official list of “On hand overseas in all theaters” from beginning to end, but there were far fewer of them than of its three peers. Yes, many were built, but they weren’t out there doing much of the fighting.