个人意见未必适用于各人口味, Karen刚回来, 肯定有她的意见和推荐.

Wine aficiandos and museum junkies like me would certainly have different recommendations. It depends a lot on what you like and want to get out of the trip and how much time you have.
如果没有意外, 我打算明年回去Florence呆一周.
I had more time in Rome last year than in Venice and Florence, but really too short in every city; so I can hardly give well-rounded advice.
If you are a history buff, definitely Rome is best. From ancient Rome to Renaissance to modern time, the city is huge, lively, bustling with energy, and provides diverse entertainment. You are never bored. If you are crazy about art, Vatican Museum alone can keep you occupied for weeks.
Venice is my least favorite of the three. Yes, one MUST see Piazzia St. Marco and the basilica, but the island is small and crowded with tourists and tourist shops. It warrants a short trip of just 1-2 days, unless you are going for the Carnival.
Florence is my favorite. It seems to have the best food and the best atmosphere. The old town is small enough to manage on foot, but full of Renaissance arts and history to keep your mind busy.
Venice is 2-hour away from Florence by train, so it would be easy to hop there for a day from Florence.
To me, Rome is like London. You get dazzled, run around till you drop, and collect a lot of cool things to talk about. It is like a fantastic amusement park--you come, you go. All the time it is around you on the outside. Florence, OTOH, feels more like Edinburgh to me. I have an intimate resonance with the place, a secret chemistry, as if I could have been born there or die there, or call it home. They are cities in which I can share a quiet moment with. I have a place *inside* me for them.
<shrug> That's just my personal subjective opinion. Your mileage may vary.