Skiing, skiing, skiing. I've skied Breckenridge, Winter Park, Copper Mountain, Steamboat, Crested Butte, Keystone....they're all awesome. Don't ask me for a comparative review, however, since I started skiing when I was 5 or 6 and don't remember most of the differences between them. They kind of run together in my memory.
Summer? Hiking, hiking, hiking. Horseback riding, camping, exploring waterfalls, spelunking.
Colorado's just one big vacation destination to me. I'd probably be willing to pick up and move there, though, if I found a good job.