Topic is best planes of WWII. There was no criteria other than that which I saw. So my answer is the C-47. Far and away did more for the war effort than any other plane. Was used by every Allied nation in every single part of the world doing the real work of any large scale war. The existence of the Goon and its versatility probably shortend the war by a year or more.
Not discounting the value of the C-47, but how does that make it the best? How long would the war have lasted if there was no B-29, No SBD, No P-51, no Hellcat or no B-24? The B-24 was very important to closing the Atlantic gap, pivotal to winning the Battle of the Atlantic. There were several critical aircraft that made a huge difference in the war. If there were no C-47, there was the C-46 and C-60. Versatile cargo aircraft were very important, but could not survive in a combat zone without complete air superiority. How does one gain that, with transports?
Best implies that it was superior to all others. Who is the best QB in the NFL? Who is the best starting pitcher in MLB? those can be more easily quantified, but still there will be no universal agreement. Want to argue most important? That's where the C-47 is a competitor.