Best for what?
I'm inclined to agree that the C-47 was one of, if not the most important tactical aircraft in WWII. It is still in service, it is still super easy to fly and maintain, it can land just about anywhere and it was the backbone of tactical ground logistics and supply - plus medevac - plus airborne infantry operations.
Considering fighters? I would tend to think along the lines of point interceptor-but lacking footprint; best air superiority fighter with large footprint; best fleet/land all around capability.
I lean toward the P-51B/C as it was the fighter that permitted USSAFE the ability to conduct truly strategic operations that had been stopped in fall 1943 because the 8th AF could not defeat the LW nor destroy critical industry un-escorted to deepest targets. Given Only one choice for a fighter aircraft for AAF to perform all missions, the 51B/C/D/K would have been the choice - and was, Post VE Day. Extreme range combined with excellent performance envelope (better, as good, nearly as good in all dimensions as any other piston engine fighter in WWII), low cost, producible, easy to train, low cost to operate.
Someone above made a comment that it did pretty well with "10:1" advantage. It, in fact, operated from December 1, 1943 through May, 1944 at a decided Disadvantage in numerical superiority as the few groups were diluted further by the necessity of performing target escort for 30-35 BG attacking distributed targets - enabling the LW to achieve numerical superiority at any point of attack.
The late model Spits could easily out-turn the B/D and outclimb them also, the 109K could outclimb and marginally out run it, the 262 was far superior in speed - but none of them could operate deep in the other's territory and do all things well at the extreme combat radius of a Mustang. This is the primary reason Gunther Rall thought the P-51 was Allies best fighter.
The Joint Fighter Conference - Patuxent River Oct 1944 when RAF, AAF, USMC, USN and Civilian pilots evaluated the P-51, P-38, P-47, P-63, F6F, F4U, Spit IX, XF8F voted the P-51D as the "Best Fighter below 25,000 feet" and the P-47D as "Best above 25,000 feet". One can only speculate what the outcome would have been had the Axis powers competed at the Conference.
Best Bomber - B-29 but most Important is up to debate because the Mossie, Ju 88, B-17, B-24 and Lancaster all have solid reasons for the nomination.
Best Trainer - AT-6 although it would be a handful for Primary cadets.
