I've been to Rialto recently, since its modest reformulation from a broader Med menu to a more explicitly Italian focus. I really enjoyed it; it doesn't seem substantially different from the reliable occasion place I've loved for some years now. This is my easy favorite of the three.
The Fed is a place I've found hard to love: it's one of the most expensive restaurants in town, but has had a revolving door of chefs since it opened. Enjoy your last $400 meal? Sorry, time to roll the dice on a new kitchen and concept. The current chef also ostensibly runs the kitchens at Mistra and Sorellina. That suggests that it will be good, consistent, and about 20-30% more expensive than it should be. I haven't been since this regime took over. Don't get comfortable with it, though: I've heard it's about to be completely redone again as yet another high-end steakhouse. Joy!
I haven't been to Excelsior upstairs in a while, but my experiences there have been pretty awful. It's one of the more hideous high-end dining rooms in the city, loud and cramped besides. I have yet to get a meal that justifies its $40 entree prices or exorbitant wine markups. Its predecessor Biba was long one of my favorites, so maybe I have an unfair bias against the current concept, which I find dull, sort of a luxury chophouse with some bare hints of its owner's former innovativeness. The bar on the ground floor remains a decent place for high-end nibibles and drinks, if you can ignore the Fellini-esque hookup scene (think young gold diggers of both sexes and suspiciously tight-faced wealthy oldsters).