DC-3 just for the sheer number produced and its longevity.
The DeHavilland Comet: Worlds first jet airliner.
The Boeing 707: 'nuff said
Airbus A300: First widebody twin. Paved the way for Airbus.
Boeing 747: Again: 'nuff said.
Boeing 737 (all series 100-900) second only to the DC-3 as far as ubiquity and number produced.
Concorde: Financial flop, but nevertheless the only supersonic airliner in regular airline service.
Boeing 727: Opened the door for all small/medium range jets and airports. Was arguably the leader until the 737 surpassed it in number built, and few...very few airlines over the years did not at some point operate the Boeing TriJet.
Airbus A320: Worlds fastest selling jet airliner. More than 600 were sold just within its first three years of service. You may love or hate Airbus, but that kind of sales record commands respect.