First of all, it is in Tuscany, possibly one of the most beautiful places in the universe. It is on a human scale, everything is more or less within walking distance. It is the birthplace of the renaissance and genius after genius after genius, who have all left their impressions and legacies on its fabled streets and nearby hills.
The art, the cuisine, the architecture, the people…the ladies; all of the best looking women in the world make their way to Florence. I never fell in love with so many Italian, German and Scandinavian ladies.
But go off season…April to mid-June or September through October. If you go in the high season, I would recommend spending some time in Siena which is very close by (also stunningly beautiful and not as many tourists. There will be music coming out of every window; they have an internationally renowned summer music program and language school).
So many beautiful cities in the world, but for me, Florence stands slightly above the rest. Oxford is a strong distant second, tied with Heildelberg. Hey, I know what I like.
I liked Florence, but I loved Venice. easy train ride between the two, as well. I went a couple years back, just before the epic flood in Venice - my hometown newspaper had a pic on the front page of some guy wakeboarding across Piazza San Marco. yikes.