Current record holder for world's longest flight (measured in nautical miles) is Qantas operating Sydney-Dallas Fort Worth (8578nm) with the A380.
On March 1st, Emirates will take that title away when the first Dubai-Auckland flight (8824nm) takes flight operated by a 777-200LR.
More detail and analysis of these ultra long haul flights here:
http://airwaysnews.com/blog/2016/03/...longest-route/
A block time ("butt in the seat time") of 17 hours and 15 minutes.

Funny enough it is actually second in this ranking, with Emirates own Dubai-Panama City flight having a block time of 17 hours and 35 minutes (although the distance flown is shorter at 8588nm).