I'd have 2 or 3 on the short list.
1: Bellanca Aircruiser/Airbus. The flying W. Nearly 4,000 pounds in useful load. Never really had a chance within the United States, due to the ban of single engine airline operations as it came about.
2: Lockheed Constellation. One of the coolest old airliners ever. I've been fortunate enough to see one, in Kansas City.
3: Saab 90 Scandia. European competition to the DC-3, in a way. Not very many made, and only 1 around any more (not flyable).