Only one? then would have to be the Wright Flyer two seater of 1907. Showed what was possible and the grandaddy of all that followed.
For greatest impact C-47. All the other P- this and B- of WWII (and since) were/are marginally relevant fluff.
For greatest impact C-47. All the other P- this and B- of WWII (and since) were/are marginally relevant fluff.