Heathrow (LHR) - 20 minute Heathrow Express train from Paddington (which is a 15 min walk from our house). *We prefer this airport! City (LCY) - 45 minutes on Tube & DLR Train (doesn't include walking to tube or transfers)
Gatwick (LGW) - 30 minute Gatwick Express train from Victoria Station (which is about 25 mins by tube, but you have to change lines)
Stansted (STN) - 50 minute Stansted Express train from Liverpool Street (which is about 30 mins by tube)
Luton (LLA) and others - avoid if possible!