I am looking for a recommendation for a reliable, comfortable Denver airport to Vail transport?
@sander CME (Colorado Mountain Express) is pretty much your only option. You can hire a private car if that's an interest.
How many people? If only one or two, CME makes sense. For more, a rental car will probably be a better deal. You can charter a van from outfits like Summit Express but its not cheap and only makes sense for a group.