The best fighter of WW2 is undoubtedly the Me-262A-1a, but unreliable engines, low fuel supply the few numbers available made sure it didn't have a chance to turn the tide of the war.
In terms of piston engined fighters the Ta-152H-1 takes the prize hands down, but again low fuel numbers also made sure this baby didn't seriously affect the war.
Next comes the FW-190 Dora-9 which IMO was the best series produced fighter of WW2, featuring much better maneuverability than the P-51, P-47 P-38, its main rivals over Germany, as-well as being faster at most altitudes. But yet again it was out-numbered in the air and fuel was low.
Even so the only aircraft which could've turned the tide in favor of the Germans was the Me-262, had it been available in numbers by the beginning of 1944 as-well as granted the fighter-role by the Führer by this time then it could've turned the tide. Could it have won the war for the Germans ? No I don't believe so, esp. not with Hitler in command, but it would've prolonged the war to a point where an invasion of Germany would've proven way too costly and other more drastic measures had to be taken - so lets consider ourselves lucky that didn't happen.