I think one of the reasons everyone is talking past each other here is that the OP wasn't specific enough.
You don't have that many generalized solutions in a single airframe. Even the P-51 had it's limitations, the Spit, the Zero, the Fw 190 and the Bf 109 certainly did as well.
First, I'd stipulate, feel free to mix Anglo and American types since they did historically, but stick to historical aircraft, no 'what-ifs'.
And then maybe it makes more sense if you break it up into specific real-world missions-
- Best escort fighter for naval dive bombers - Hellcat or the Corsair (I'd say Corsair for it's higher speed and other qualities but Hellcat is probably more stable of a design earlier on)
- Best long range naval / maritime escort (ala Malta zone etc.) - Beaufighter
- Best high altitude / long range - Probably P-38 in spite of all the problems. That's the one they used anyway for escorting the B-24's and B-17's at least down in the Med*.
- Best short-range land based / tactical escort for fighter bombers or medium bombers - Spit IX or P-47D (I'd go with the Spitfire IX)
- Best medium-range land based / tactical escort for dive bombers, fighter bombers or medium bombers - P-40F, P-51A or P-47D (I'd go with the P-40F)
- Best low altitude / long range escort (like to escort a low alt Mosquito raid) - P-51A
- Best low altitude / short range escort (escort Hurri IID)- Spit IX LF (clipped wing)
* I suspect Corsair would be a good substitute with extra fuel etc.
S