Chuck Yeager flew all the fighters after the war, and says the P-51 was the best; the only one even close was the Focke-Wolfe(sp?Wulf 190.
My personal favorite since I was a kid was the F4U Corsair. Anyone ever watch Baa-Baa Black Sheep?
You can't disregard the P-38 since the top scoring American ace of all time, Dick Bong, flew one.
Spitfires are pretty, and famous because of the Battle of Britain, but are outclassed by the Mustang.
The F8F Bearcat was probably the ultimate in piston engine fighter development, it broke all the performance records as I recall, but unfortunately came out at the end of the war and didn't see combat.