For sheer volume, Vail is the biggest. You could be there a week and never ski all the runs. Lived in Vail for 3 years and would get 150+ days a year on it.
I lived in Avon for a year and skied Beaver Creek all the time, we used to hike the "Bald Spot" off the Grouse Mountain lift (10, I think) and board the heavy powder. About the best anywhere in CO. The Birds of Prey run is just amazing. Built some houses in Bachelors Gulch, so skied it a bunch too.
If you are a boarder then Copper is really good, I think the nightlife in Breck is the best and it's a very solid mountain. Keystone is on the more beginner/intermediate side for my liking. If you are really good and like a challenge then A-Basin is the place for you. I really like Crested Butte but there is not much to do there and Gunnison (20 miles away) isn't much better.
Aspen is over-priced and stuffy (but the don't allowing boarding, or at least didn't use to)