Well, if you are talking in history, it was probably LAX-SYD with the 747SP. As far as now, BKK-FCO sounds good to me. I would guess that before they switched to a 744, QF's 743 BNE-AKL-LAX was pretty long.
Mangeons les French fries, mais surtout pratiquons avec fierte le French kiss
I believe UA used to put their 741s on the LAX-NRT routes, and counting in the 747SPs, TWA operated their 747SPs from LAX-Tel Aviv. Also, TWA and Pan Am used the Boeing 747s on LAX-LHR, STL-CDG, STL-FRA, etc.
QF's 743 never flew AKL-LAX regularly, there were a couple of 743's used for some reason back in 2002, no idea why though. QF never flew classics on AKL-LAX as far as i'm aware, I think they started MEL-AKL-LAX in 1994 with a 744.
LAX-AKL is 12:45 which NZ did daily with 742's which is fairly long!