I just got back from my first CO trip last week. We got 6 total days on the snow-- we hit Vail, Keystone, Beaver Creek, and A-Basin.
Vail was a blast with fresh snow, but wasn't great without it. Lucky for us, we had a local buddy who took us to some of his favorite stashes in the Back/Blue Sky. We found great tree runs at Blue Sky that stayed fresh for a couple days.
Beaver Creek was a total waste of time. It was a pain in the ass to get around the mountain, the conditions sucked, and the whole feel of the mountain was wack. It's a whole 'nother level of rich people.
Keystone's groomers were a ton of fun. Long, fast, bomb-able. And you can actually ride fun trails between the areas of the mountain, as opposed to being on a cat track. I had a lot of fun on that mountain.
A-Basin was our favorite day of the trip. They had just gotten 8" the night before, and it was empty on the hill (it was a Tuesday). We lapped untouched powder for hours. It feels like a small mountain when it comes to lifts, facilities, etc., but the terrain itself was pretty great. Montezuma Bowl has fun stuff to explore if you're up for some walking/traversing.
I'm obviously no expert on Colorado terrain, but I would say it all depends on the weather. Without fresh snow, I'd personally stay at Keystone and tear up the groomers. With new snow, A-Basin is excellent, not so sure how it would be without it. Vail is so huge that you can always find something fun to do. Stay away from Beaver Creek.