IMHO:
SIN, CPH, ARN, DXB, HEL are nice, clean airports.
AMS and BRU are quite good, too.
Always surprised that AUH is a bit of a dump.
Didn't think BSB too bad - just has rather limited facilities and staff with a poor customer service ethos.
Why is it most UK airports are pretty tatty?
The drop-off/pick-up points landside at MAN are littered and pocked with chewing gum.
The waiting area @ MAN T1 departure lounge is an absolute disgrace (despite the appointment of an oddly-titled "Terminal Appearance Officer").
Toilets in MAN & LHR would disgrace the Third World.
For God's sake, someone should get rid of the carpets in the terminals of some UK airports. They look awful.
As for LHR - a national disgrace.
CPH - coffee at Cafe Karen (Dixon?) is some of the best I've had anywhere.