The best one that I've used is Hong Long.
The best large airport in Europe is Amsterdam.
Going down a tier, I'd say Helsinki. Athens isn't too bad in terms of building structure, but the organisation of it could be better.
There is a lot to be said for smaller airports though. As good as Amsterdam is, I would happily use Rotterdam instead if the flight was available. Getting off your plane and being out the front of the airport within 5 minutes is a great experience. The only drawback is that there isn't much to do if your outbound flight is delayed.