Another vote for SIN because as already mentioned it takes but 15-20 minutes from gate to taxi. Moreover, it's super-efficient for connections/transfers because arrivals, departures, lounges, Internet cafes, duty-free shops etc. are all on the same level. HKG is a beautiful airport but not efficent because departures and arrivals are on different levels so even if you're arriving at and departing from the same gate you still have to go through the rigmarole. Having said that, SIN's aesthetics leave a lot be desired but we're talking "efficient" here, right?