By far my favorite city has been Vienna. The art, the architecture, the cleanliness, the ease of commuting (very pedestrian- and bike-friendly place)...
I stayed in a hotel near the city center and walking to my destinations was a breeze. So many coffee shops, so many historical buildings, I loved everything about it.
I'll definitely come back again.
Bucket-list for next visit: stay in other cities like Salzburg.