In my experience, no contest: Changi Singapore.
Immigration a breeze, baggage reclaim exceptional, 5 mins from getting off the plane your bags are ready waiting for you, shops ok, sign posting great, clean (of course, littering is a hanging offence here), transport to and from the city always plentiful and a fair price, no unmetered taxis etc.
Benchmarks are:
BKK (old and new) both awful, nothing good about them
KUL ok but dull
HKG (new only) also ok but confusing layout
CGK, actually not that bad.
DPS, come on guys this is the tourists' first look at Bali?!?!
HKT, as above but substitute "Phuket". Hire cars an utter rip off.
Manila, least said the better
Taipei, ok but lots of walking
NRT, pretty bad
CDG, needs pulling down
PER, a bit small so should be better than it is, immigration is a nightmare
MEL, drunk on arrival and departure, Lions 2001 Tour so cannot comment
SYD, average
Hanoi, good airport, shiny and new
ZUR, good access for the trains but pretty unmemorable
CPT, not bad. Immigration ok, baggage reclaim ok, good signposting
STN, ok but overworked now, too many LCCs and plebs in shell suits
LHR, welcome to the third world. How bad is this place in every respect??