Florence
Florence is almost too beautiful. It is also very crowded. I've visited Florence twice, but on both occasions I failed to get into the Uffizi. The first time it was not yet possible to book a ticket online and I didn't feel like queuing for an hour and a half so I visited the Palazzo Pitti instead. The second time I had a bad case of food poisoning on the day that I'd planned to visit the Uffizi. Maybe one day I'll get a third chance. Update: I finally did!
Left: Campanile di Giotto; Right: The Rape of the Sabine Women by Giambologna at the Loggia dei Lanzi