Are you aware of the location of the St Regis in Rome? We almost stayed there, and after walking by once we were in town, we were *very* glad we had chosen to stay elsewhere, near the more active areas. We stayed at a somewhat lesser hotel, and although we thought it was great (terrific location), we definitely wouldn't recommend it to someone with a short list like yours.
We loved the St Regis in Florence. They surprised us with a room with a view of the river and of the "the bridge".
NICE surprise!
We considered the FS, but chose otherwise because of location. However, the St Regis also wasn't right in the middle of things, but it was a more pleasant location than in Rome.
In Venice, we had a suite right overlooking the Grand Canal, and to us, that's part of "being there"!
We were at the Bauer Il Palazzo (not the "Bauer Hotel", which is not directly on the Canal, although they are right next to each other).
Again, with your list, although we'll return to the same next time, I'd probably recommend something like the Gritti for you.
GC