Best as in grueling and ridiculously long?

Singapore - Tokyo - Los Angeles - Lima - Santiago de Chile - Puerto Montt, no layovers. Can't remember how many hours it was, something like 46 or 48, I think. That was more than 10 years ago.

Last July I did a Barcelona - Newark - Toronto - Chicago - Portland - Sacramento - LA - drive to San Diego and back to LA - Newark - taxi to JFK - Madrid - Barcelona in 13 days. Not fun either.