Well, if you're going to Montana, you might as well make the trek to Whistler Blackcomb in BC.
Colorado is better than Whistler Blackcomb. At least when I've gone.
Colorado is world class, period. Vail, A-Basin, Breck, Copper, Keystone, Beaver Creek, and Loveland are all within 40 minutes of the Dillon/Silverthorne area. You just aren't going to get that kind of selection anywhere else.
Idaho is really good too. Bogus Basin, which is a little resort near Boise is one of the best under the radars places I've been. We went the year of the Humanitarian Bowl, and then the next. Fantastic snow. Sun Valley is good too, but very isolated.