An unspectacularly flight from FRA to SIN in November 2012 using an A380 of Singapore Airlines. It was about 12 hours. Ok, not the longest offered in world, but my longest up to date, even longer than my many done flights to Namibia, FRA-WDH and back (10 hrs).
The longest nonstop flight ever was offered by Singapore, too: SIN - JFK, straight up to north, crossing the northpole and then southwards, about 18 hours. If I have enough Lufthansa Miles, I'll do the nonstop flight FRA - EZE, this is the longest nonstop flight offered by Lufthansa, about 14 hours