For Denver, easily Great Divide. Outside of Denver, I have to say Odell.
Odells is great. I wish we could find their beer further out east. Every time I pass through CO I always bring back their 90 Shilling and something new. I remember when they first opened in 1989 at the grain elevator. I loved walking in there to get beer. I don't recall they even had a cash register and as long as we looked old enough...
Wild Mountain Smokehouse and Brewery in Nederland is definitely worth checking out if you're ever up there. I have yet to try a beer from Avery or Oskar Blues that I didn't like. I'm surprised that no one's mentioned New Belgium...ok maybe not.