Checked my logbook while the weather is sh1t!
Quickest:
April 2008, BA, B747-400, G-BYGG, LHR-JFK, 6:56 block time, so if 20 minute taxi out and 10 minutes taxi in, just under 6:30 airborne
Longest:
March 2006. BA. B747-400. G-CIVR, LHR-JFK 8:35 block time (incl 15 minutes airborne holding)