Re: For Italian Posters...
is your choice dictated by academics (good courses/university) or just by fun/travel? In the first case you should check the faculty you will be in, but in general of the ones you mentioned, the best universities are in Milan>Rome/Torino>Siena/Bologna>Florence.
About the city itself, i'd say Rome > everything else. Florence/Siena are pretty but not that active for students, Milan there is a lot going on but it's ugly and has bad weather.
For traveling in Europe, Florence and Milan have the best train connections, Milan and Rome probably the best connections by air.