My favourites:
Milan: Le Meridien Gallia (closed now). Handy location, and good upgrade for me. Sheraton Diana Majestic wasn't bad either.
Paris: Prince de Galles. Nice location (near Arc de Triumph). I like its traditional decor.
Westin (better rooms) close behind, also good location, better if going to Palais Garnier. Had a very impressive upgrade despite being on free night.
London: Park Tower. Great location, and no 'horrible' rooms, although nothing particularly outstanding about the rooms. Services at the reception can be a bit distant but never rude.
Warsaw: Le Meridien Bristol. Excellent service and terrific location near the Opera House.
Athens: Grande Bretagne. Beautiful hotel, great view of Acropolis from the restaurant, lovely service (impressed that the manager recognised me and greeted me by name), great upgrade.