LGWAlan, so is HEX, but both GEX and HEX are well-equipped for luggage-wielding travellers. Currently East-West and North-South connections are not luggage-friendly, so more luggage-friendly transport would make things a LOT more attractive for people (at a price).
Airtrack would probably be just as well-equipped as HEX and GEX are because it again is designed for travellers. I won't talk about the Stansted Express - its interior is, compared to the other two, a disgrace. Luton doesn't even have its own express service, you end up on a grubby semi-fast Capital Connect if you're lucky.
When CrossRail finally takes off, STN would be connected to CrossRail at Liverpool Street, requiring a change at Farringdon for LGW (40 minutes) and LTN (50 minutes). Heathrow would be a straight run of 35 - 40 minutes on CrossRail from Liverpool Street.
S.