People often forget that the P-51D didnt come into relatively late in the war, and was most significant because it could carry the fuel to escort bombers deep into Germany. The P-47 came out earlier and was excellent in close-air support as well as air combat. However, when looking at the entire war from 1939, the BF-109 was arguably the best. Only the Spitfire could match it until the P-47's arrived. As for the Pacific theatre, the Hellcat gave the U.S. a fighter thatc could hang with the A6M Zero and changed the course of the air war, so it takes the cake in the Pacific.