Mitsubishi A6M: It gave the Japanese the confindence in the air against anybody else, and that helped infuence their decision to attack the USA. After initial resistance due to the A6M's lack of manueverability Japanese pilots came to regard the Zero the way Samurai thought of their katanas. Jiro Horikoshi (Mitsubishi's lead designer) was more famous in Japan than Reginald Mitchell was in the UK.
Supermarine Spitfire: It sold hope and eventual victory to the British people in a way that the Hurricane could not even as the Hurricane was doing the hard work. Morale had more to do with winning at that point than battlefield performance. The Spitfire also soldiered on, sucessfully excepting upgrade after upgrade, and remained a first rate front line fighter for the entire war.
Messerschmitt Bf109: It lead the fighter aspect of the early German victories, proving superior to everthing it faced until the Spitfire, and then it was so close that there isn't any agreement as to which was actually better. Later superceded, but never out infuenced, in the Luftwaffe's effect on the war by a fighter the Bf109 was the fighter of choice for the great German aces.
Grumman F6F Hellcat: This is the fighter that decisively and irrevocably turned the air war against Japan. It wasn't until late 1944 that Japanese fighters appeared that had the advantage on the F6F, and by that point quality of both manufacturing and pilots was down. It was too little too late.
Yakovlev Yak 9 and 3: The Yak series held the line against the Germans and then pushed it back. The Yak 3 being so effective at the low altitudes of the Eastern Front that the Germans tried not to engage it without substantial altitude advantages. Produced in even greater numbers than the Bf109, the Yak carried the fighter effort for Russia.
