Single engine prop -- easy: A-1 Skyraider.
Range, endurance, payload, and even killed a couple of MiGs in Vietnam.

Having talked with guys who flew the A1's and variants, it, in the opinion of several vetran pilots, the best all around single engine, prop driven aircraft ever produced! It could carry a bigger bomb load than a B-17 or B-24. It could carry up to 14 5.5 inch rockets as well. Then there was the damage issue! While this aircraft was the largest single engine aircraft ever produced in large numbers, it could take a hammering from ack and 37MM and keep right on flying! They were called "Sandy" in the Vietnam war and many a foot solder was glad to see them over head as they could loiter for hours if need be! The only aircraft ever built which could carry more payload weight than its empty weight! Empty weight, 11,968 lbs, max allowable takeoff weight, 25,000 lbs. And if need be, could do ACM as proven by 2 Navy pilots During the war, U.S. Navy Skyraiders shot down two North Vietnamese Air Force (NVAF) Mikoyan-Gurevich MiG-17 jet fighters: one on 20 June 1965, a victory shared by Lieutenant Clinton B. Johnson and Lieutenant, junior grade Charles W. Hartman III of VA-25; and one on 9 October 1966 by LTJG William T. Patton of VA-176.
When you ask "which is the best fighter" ever built, prop driven, I would have to say the P-51K, but when you ask which is the best "fighter-bomber" ever built, I rest my case on the A1!