As far as terrain goes Aspen pretty much kicks the pants off of anything in Summit and Vail Valley. As far as the Apres scene goes Aspen pretty much kicks the pants off of anywhere except Whistler.
Crested Butte is pretty much the steepest overall resort in Colorado. Probably my favorite when I am spinning lifts. It is also damn near the rockiest and unless they get hammered, December is awfully early to go there. Core shots are almost a given there. The town is great, it is low key and very mellow. Not much of a party scene, but you can probably find a place or two to entertain you.
The thing is this La Nina year might a favor Summit/Vail, Winterpark, and Steamboat more.
Joel Gratz, our local guys who runs Coloradopowderforecast
put up a nice review
on what he is seeing for this La Nina winter. Of particular interest is the page
on Colorado and Utah. The further south you go the worse your chances are. Also, Tahoe might get skunked this time, and Mammy does not look like a good bet. PNW, Northern rockies (Montana, Idaho, Wyoming) could just get nailed.
One thing about Joel that I've noticed is he's one of the best forecasters in the business. So sure it's looking into the crystal ball, but he's about as big of a weather geek as their is when it comes to snowfall.