There's a great thread over at
Airliners detailing the world's longest flights. The top five is as follows:
8285nm SINº-EWRº Singaporeˇ 345x7
7621nm SIN-LAXº Singapore 345x5
7454nm SYDº-DFW º (see footnote †) Qantas ˇ 74Ex7[5]
7334nm ATLº-JNBº Deltaˇ 77Lx7
7246nm DXBº-LAX Emirates ˇ 77Wx7 77Lx7