For me it depends on several things...
1. Are you a skier and don't want snowboarders? - Alta
2. Big Mountain, lots of snow? - Well, any really, but I still say Alta
3. Big jumps and a fun Terrain Park? - Brighton
4. Silky smooth groomed trails? - Deer Valley
5. Night Skiing? Brighton, Park City
I've never skied Snowbird, or the Canyon's, or anywhere else in Utah other than the Cottonwood Canyons and Park City area, so my opinion's are incomplete at best, but that's my $.02.