This one sure is a tricky one.
I agree with those who has observed that the tide didn't turn on one spesifik day because of one specific air action. But here goes.
The metafor with the turning tide refers to the high water mark. On the maps that can be said to have occured in late 42, though Germany in desember 41 had been closer to Moscow, and the Japanese would advance considerably in china in 44. But by and large, the push back or stalemate, in most thearters, started then.
This high tide is visible on the maps, and that is a point. Airpower did not decide the outcome in splendid isolation. If I have to choose one plane, I'd pick the P-40. I have to admit I was never that fond of this plane. To me the air intake on the later models looks just wrong, no matter how well it lend itself to shark mouth motives. The earliest models i consider to be far more harmonious in outlook. But i digress.
In my opinion the impoertanse of the plane is that it was there from relatively early on, and it was available in numbers. I will not go into the debate whether it was the best US fighter at any time, but it had few rivals, maybe the P-3...8? but it was more ready than this, and it served in all major theaters except northern Europe. Had it not been available, either less or worse fighters would have had to be used in the Pacific, East Asia and North Africa in especially 42. I'm not damning with faint praise, I just don't think it was the all round best fighter in any airforce at any time. Possibly excepting the Chinese.
As the tide was on the ground, it is impoertant that it lent itself well to ground attack. again maybe not ideal, but it did make for more adequate airframes to throw in during the important years where the axis were slowly blunted and the offensives ran out of steam. Even with a few thousand less allied fighters of decent ability around before the end of 1942, the allies would still have won the war, but the tide may have turned a little later. Of course other planes contributed as well, but few of them in as many theaters. The Hurricane may have served nearly as widely, and at least up to a point in larger numbers (I'd like to know if there were more operational Hurricanes than P-40's around in november 1942?), but I do consider the P-40 the superior plane. I know, only up to a point. The allies needed all the fighters they could get, and the lack of any (or at least all that served in meaningful numbers) would have been sorely missed.
But all in all, for the reasons here stated, I consider the P-40 a very good candidate.