Without a doubt the DC-1 to DC-3. They proved that economic and reliable airline transport was really possible for the first time and were clearly the precursor to all modern air transport.
YO...just back from a couple of weeks away exploring the rural hamlets and small aerodromes of my native land (New Zealand) and photographing Gulls in flight. So it is good to see another small voice in the wilderness nominating the DC-3. Ecktually, upon reflection, I place the Boeing 747 and the DC-3 as equally significant in the furtherance of safe, reliable and affordable air transportation for the great unwashed. So one can only hope that these two great aircraft will be high on the listings when it comes time to cast your vote. If the quest was for the most elegant airliner of all time, I would come out strongly for the Lockheed 1049 Connie...a pure work of art. As a mere lad of 26 I was indeed fortunate to have flown many hours in an old Redifon non-moving simulator and, when it was not busy during the weekends, revelling in poling it around and down the Melbourne ILS solo. It had no visual capacity, but it was sure a hell of a lot of fun, if just a tad busy on one's own...but I digress...sorry about that troops!
Whoops....been away from the keyboard too long. Should have mentioned that the Redifon Lockheed 1049 Sim. was a recently retired QANTAS unit purchased by Trans Australian Airlines (TAA) who moved it from Sydney to Melbourne and used it as a general purpose four-engine trainer for we chaps who were operating DC-4, DC-6 and Vickers Viscount. I was a newish Viscount driver when they were still on main trunk interstate services. Oh...happy days (and nights).