I'd vote for Wheat Ridge Cyclery, good selection and good service. Bike Source is a huge store, but they don't get points for customer service in my book. They totally turned me off when I was ready to spend the cash. I've heard good things about The Bicycle Pedal'r but haven't been there myself. Treads is another good one, but smaller.
http://bicyclepedalr.com/site/intro.cfm
http://ridewrc.com/site/page.cfm?PageID=311