Douglas C-47 Skytrain
The C-47 was the military version of the DC-3, an outstanding aircraft that has been in service for over seven decades and just keeps on flying. It is almost single-handedly responsible for the birth of the airlines, but don't hold that against it. It was and is one of the most reliable and useful planes ever built, and served major combat roles in several wars. Its ideal blend of ruggedness, utility and performance have insured that the only real replacement for a DC-3 is another DC-3.
Cool stuff for C-47 lovers: