Tough call here. Many of the fliers in AH are fixated on pure permormance. But it takes more than high speed to make a successful combat aircraft.
I would disqualify most late war aircraft, of all combatants, for the simple reason that by the time they were deployed the war had already been won (or lost...depending on which side you were on) and their contribution had little effect on the war's outcome.
The criteria which I shall use to pick the greatest aircraft of the Pacific are these:
1. Performance
2. Availability
3. Maintainability
4. Ruggedness
5. Range
6. Firepower
7. Manuverability
Army and Navy fighters of the U.S. that were in action in the period from late 1942 through late 1944 score high in all categories. In each design were rated on a scale of 1 to 10 in each category listed, none would score lower than a five in any of them.
Because of the differing nature of the war fought by Army and Navy aircraft, I will pick a winner from each area. While the aerial war against the Japanese was mainly a carrier war, the contribution of Army aircraft was substantial and cannot be ignored.
In air-to-air combat against the Japanese, the winner has to be the P-38. It's combination of performance, manuverability, firepower, and range were well-nigh perfect for the Pacific Theater. If the ground attack role is considered then a case could be made for the P-47 being the most important army fighter. But the P-47 falls short of being ideal for the Pacific because of its lack of range, at least until late 1944.
For the Navy, there is really no contest. It has to be the F6F. No other fighter during World War II so dominated a combat arena as did Roy Grumman's portly creation. The Hellcat destroyed over 5,000 enemy aircraft while losing less than 300 of its own number to enemy action. The official kill to loss ratio was 19 to 1, a figure not within the reach of any other fighter of that period, nor of any since. In terms of sheer performance, the F4U was faster, but it's treacherous low-speed handling characteristics kept it from carrier qualifying until early 1945, by which time the F6F had devastated Japanese airpower. Perhaps the Hellcat's greatest asset was it's vice-free handling, which allowed it's pilots to squeeze every last ounce of performance and manuverability from it. Hellcat serviceability rates were consistently about 90 percent, another figure that was not within the reach of any other aircraft of that period.
For the Japanese, only one aircraft was produced in enough numbers to have a significant impact on the war. That was, of course, the ubiquitous Zero. No other Japanese aircraft was ever produce in anywhere near the same numbers as the Zero, with over 10,000 being turned out before the war ended. But it's low score in several of the above categories precludes it from being considered as the overall winner.
So there you have it. For the Army, the winner is the P-38. For the Navy, the F6F.
Regards, Shuckins.
P.S. By the way, it would be interesting to see how other readers would rate these fighters in each of the categories that I listed on a scale of 1 to 10. What are your thoughts?