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.
I'm speaking exclusively about real life, not in this game. In real life the Luftwaffe enjoyed a superiority or parity of fighter performance in five out of six war years. Of course, the one-year period they didn't is when they lost the air war over Europe, and the Luftwaffe was all but destroyed.
From the start of the war in 1939 until the Battle of Britain in 1940 the 109E and 110C ruled the skies of western Europe. When the RAF got Spitfires with the updated Rotol propeller they could match the 109E in performance. However, by the end of the BoB the Luftwaffe started receiving the first 109Fs, again achieving a performance advantage over the RAF. As Eric Brown would attest to after flying it, at this time the 109F was undoubtedly the best fighter in the world. The Luftwaffe would enjoy this advantage for more than a year with the advent of the Fw 190A. 1941 and 1942 was a bad time for the RAF and Commonwealth air forces in Africa, the Mediterranean and over the Channel. This is also the time "Jochen" Marseilles would amass his kills and certify his status as the "Star of Africa", flying the 109F.
This is also the time period where Japan and America entered the war. The best fighter in the Pacific at this time was the Zeke; the famous Mitsubishi A6M Type 0 Model 21. However, the 109, 190, and Spitfire were in a league of their own at this time in the war, with performance advantages that would be all but insurmountable to any Japanese, American or Soviet fighter in service. This is also the time when the numerically superior Red Air Force was all but destroyed by the Luftwaffe.
In the summer of 1942 the 404 mph Merlin 61/63 powered Spitfire IX achieved parity in performance with the 405 mph 109G and 408 mph 190A, ending the "twittler scourge". This parity would last for a year with the Luftwaffe scoring more kills over France and the Channel mostly because of tactical advantages, rather than a performance advantage. At this time there was little to choose between the three primary fighters in Europe, all having only slight advantages in some areas of performance at narrow altitude bands.
In the closing months of 1942 the Americans entered the war in Europe with their early models of the P-47C, but they fared poorly against the Luftwaffe. The P-47 would not become a mature weapon system until the D model.
This period of relative parity in performance would last until the fall of 1943 when the RAF got the excellent Merlin 66 powered Spitfire LF Mk IX (comparable to the Spitfire XVI in AH). The Americans had also started flying the P-47D. Although still a flawed design for high altitude combat, the P-47D would hold its own against the Luftwaffe using tactical advantages. This marked the end of Luftwaffe's superiority/parity in performance and the beginning of the end of the Luftwaffe itself. Still, the war-year 1943 would end with the Luftwaffe having won significant victories in the air war over Europe, destroying the American 8th Army Air Force three times over. Though it is arguable that this achievement would not have been possible without the insane American belief in the self escorting bomber concept.
With the advent of the excellent escort fighter P-51, and a completely revised Allied air war strategy the first half of 1944 saw the Luftwaffe swept from the skies of western Europe. Out performed, out trained, and most of all out produced, the Luftwaffe could not hope to win. The final nail in the coffin was when a desperate Luftwaffe closed down its pilot schools and sent the instructors to the front. By June 1944 the Allied air forces flew unopposed over western Europe.
Perhaps ironically, this moment in time also marks the end of the Allied superiority in fighter performance. Late spring, summer and fall of 1944 saw the introduction of several important German advances in fighter performance. The 109G-6/AS, 109G-14/AS, 190D-9, 109G-10, 109K-4, achieved performance parity with the Allied fighters roaming the skies over Europe. Some, like Eric Brown, would argue that the 190D-9 was the finest piston-engined fighter of the war.
Then there's this...
The supreme fighter of the war. The ne plus ultra. The revolutionary fighter that on 26 July 1944 rendered every other fighter in service anywhere hopelessly obsolete. The fighter that was hand built in forest clearings and railway tunnels from the crude materials that were available in a war-torn Germany. The fighter that was flown by a handful of veterans against thousands, THOUSANDS of Allied aircraft. Those who survived getting airborne from their vulched and beleaguered airfields were untouchable. Untouchable! They ripped through Allied bomber formations at will. Leaving the escort fighters behind with "a tap of the throttle".
So yeah... As far as fighter performance goes I say German > British > American > Soviet > Japanese.
