I never liked these kind of debates because there's really no right answer. It's not a sport with a fair score. There's just too many variables that changed often in the war.
As for my opinion, why does it have to be just one? All the planes in the theater contributed, some in huge ways and most relyed on other's success.
The Spitfire couldn't have done it without the Hurricanes and vice versa.
The B-17 couldn't have done it without the Mustangs.
The Mustangs wouldn't have been produced if not for the Hurricane's and Spitfire's successful track record using the Merlin.
The Thunderbolts and Lightnings paved the way for the Mustangs in the escort role.
The Bombers like Mosquitos and even the Mediums & Heavies couldn't bomb as effeciently without the photo recon planes like the Lightnings and Spitfires.
The Typhoons and Thunderbolts wouldn't have been so succesful in the ground attack role if not for the Spitfires, Mustangs, and Tempests clearing the skies of German fighters first.
To me, the Mustang wins, not in saving Britain...but in saving me! That's because my grandfather was a B-17 radio operater. He once told me he would not have made it if not for the Mustangs protecting him over Berlin. Therefore I wouldn't exist...so I'm a bit biased. :salute: