For those of you who know how I feel about Mustangs, my choice of the F4U may have been a suprise but RenRich put very strict guidelines around the choice.
WWII offensive airpower in fighters was about two dimensions - agility at all altitudes a long way from home and ability to punish and survive in Fighter Bomber role. With Radar technology it was still a nice attribute to engage enemy formations a long way from the target if possible.
My second choice was the P-51 with growth through the P-51H or the Fw 190with growth through the Ta 152. Even if Mustang had started like with Merlin I would still choose the F4U for the reasons below
The P-38 was better in max range and ground support, less capable in high altitude air to air. The Spit for me was the best defensive fighter in continuous service but lacked range and payload to be top escort or ground support. The 109 was similar to Spit in all regards. I felt the F6F was inferior to F$U in almost every dimension.
But, in 1942 the F4U would have been fully operational as a fast, long range, fighter/fighter bomber and had the engine baseline in the R-2800 to develop an even better high altitude fighter than the P-47, would have been able to carry two drop tanks immediately, would have been as good a dogfighter at low/medium altitudes as we had in the war and superb through 24,000 feet. The F4U-4 and 5 would have been developed earlier and were faster than 51D and marginally slower than Fw 190D-9 and P-47N in speed at altitude
Obviously the USN would have had to commit to it for carrier Ops earlier, the AAF would have had them available in numbers at Guadalcanal and New Guinea and North Africa and England when 8th AF ops started there as there were no other fighters soaking up production capacity.There would have been more bandwidth from Aero and propulsion teams crafting derivatives - including possible in-line engine advances, different airfoils, lightweight airframe, etc to achieve very high altitude 'niche' Corsairs if necessary (same as Ta 152 or P-51H).
Ground Support? Already better than P-51, Spit, (and Fw 190 if range is a factor for ops). Tough as P-47, huge payload, heavy firepower either in 6x 50 or 4 x 20mm and way better than P-47 on deck in defensive mode.
And for what it's worth I think you have to include Naval air simply because that was a requirement that added a lot of structural weight to the F4U that, removed, would have increased its thrust to weight, wing loading, top speed and climb rate. Let the others add the weight to be modified for Carrier ops (such as P-38 or Fw 190D-9) to get it on level playing field?