Venice is magnificent. My time at university was greatly enhanced by having a year studying the history of Venice between 1400 ad 1700. I've only visited twice, but it is unquestionably a fantastic, unique place - so much history (my appreciation was really enhanced by having studied the place). I'm not sure what it's be like to live there, unless you're an author seeking inspiration, or a millionaire (always a good thing when looking at living in cities), but there's nothing in the original question about having to live there...
Other than that, here are my faves:
Paris - history, architecture, culture, food, french, remarkably easy to get around for a major city (especially on a Velib after a few post-prandial armagnacs...)
San Francisco - the geography, the vibe, the food, some magnificent scenery nearby, sensible climate
Hong Kong - the "coolest" city I've ever been to, the energy (this is properly a city that never sleeps), excellent public transport (cycling possibly a nightmare, however), a short boat trip from beautiful quiet islands, the food, the smell (all those restaurants)
And I don't think I've ever been to a German city that I haven't liked - Cologne, Munich, Heidelberg (or is that a town?), Nuremberg...
Honourable mention to Albi as well - small french provincial city, but a nice bit of Cathar history. Been there lots on holiday and it just keeps getting nicer. Included for sentimental reasons.
Oh, are we supposed to pick just one?