I have two nominees.
firstly, the C-47. In my opinion it probably did more to influence the outcome of the war on so many theatres of operation.....Burma, Manchuria, new guinea, Italy, NW Europe, to name just a few.
secondly the IL-2. Despite its horrendous losses and disdain we in the wwest tend to view it, it probably did more than any other attack plane, or fighter due to the impact it had on the eastern front.
Least important are any and all the fighters of any nation. They were needed and they were important, but nowhere can it be said that they won the war, or even averted defeat. The nearest that this was achieved was probably Phil sea, but even if the IJN had broken through and a surface battle ensued, the Japanese fleet was so outclassed by that stage I think they would have lost in any case.
I don't agree with the B-29 argument. The dropping of the a-bombs had far less effect influencing the Japanese to surrender than Americans believe . It had an effect, but not critical. The invasion of Manchuria by the red army was more important as an influence to the Japanese surrender. It did lead to the Japanese acceding or forfeiting one of their last two conditions for surrender, namely that the home islands not be occupied, the other being that the emperor be respected. Even here, it is arguable that the Japanese gave up their resistance to being occupied solely because of the bomb being dropped. some believed, that with the 2.5 million Russians now in the war, the Japanese at last realized that the jig was up.
The Bomb Didn't Beat Japan ... Stalin Did