My vote goes to the 707! While the Connies were graceful and elegant, the 707 put TWA on the worlds stage. For a while on the 60's and early 70's there was a TWA 707 landing/taking off from most major cities in the US and dozens of nations around the globe.
I think TWA operated a total of 88 707's at the peak of its use in the fleet. Maybe someone has the exact numbers/breakdown.
I remember reading about their first 707..flew one round trip between SFO
and Idelwild for a month( the first month) without one cancelation or problem.This happened when HH
was in a legal tuft with the banks about financing the rest of the original 707 order.
I'd also give an honorable mention to the 747, and Convair 880...both graceful airliners ..the 880 looked futuristic in its own way with the "Starstream 880"