Mine (and also my daughter's) was Pisa. What an amazing experience to turn the corner and see the Tower. It was almost surreal. Plus the basilica is amazing and SO interesting, and the baptistry was pretty cool, too.
Almost all of the excursions we did were fantastic. Our Rome excursion was great, but the Vatican museum portion was more like a cattle drive, unfortunately. Beautiful stuff to look at, but not real enjoyable.