Just IMHO, but comparing performance using only the main models of planes (ie: models in large numbers in not post mid-44) Id put the nations in this order for the follow criteria.
USA, UK, Germany, Russia, Japan (for all round fighters, if you could only pick one)
Spitfire of course is the best pure fighter, when compared to any contemporary aircraft. Although If I had to pick one plane to equip my entire fighter arm, it would be the P-51.
Speaking purely game however (MA): the best fighter in the game is a contest between the Spit 14, F4U-4, and 109K4. You might add the 51 in there if "usability" characteristics are being considered, like stability in aiming during a dive, or cockpit visability.