There used to be a British Airways flight on the Vancouver-Sydney leg, but not sure whether back then it was a 747-400 or a -200.
Other contenders: Johannesburg-Harare (Lufthansa until some years ago), Paris-Zurich (JAL in the past), Hong Kong-Taipei (CX, CI
There have been other even shorter hops, but they were all on a -200 or -300: Buenos Aires-Montevideo (AF), Kigali-Bujumbura (AF), FRA
), and my absolute all time favourite: Brazzaville-Kinshasa, operated by Sabena. Literally across the river, it's like going from Manhattan to TEB.