I would go with the DC-3/C-47. That airplane saw service in WWII, Korea and Vietnam. It is still in use today by some companies to haul cargo into the colder recesses of Canada, where jet fuel turns to jelly in the cold. When it was originally designed, one of the requirements was that it would be able to fly with one engine and land okay. To test it, they took off and flew an entire flight on one engine!
Granted it wasn't an elegant fighter or bomber, but when it came to hauling troops or cargo, it was always there and in some cases, still is! I'm a big fan of the Goony Bird.