I just moved to Denver 4 months ago....This is the best I know so far...some from personal experience other info from BMW car owners.
Gebhart, in Boulder, is pretty bad. They broke my windshield; put DEF all over the engine on a refill; and failed to put back some bulkhead parts in properly. Thankfully they will fix what they mess up, provide nice loaners, and will negotiate on the service cost. (FYI I will no longer be going back to them)
Murray in Denver, never been there but others have told me to stay away. I called them on the phone for some service info and pricing. They were not the friendliest people and refused to move on price. (I would pass on these guys)
So that leaves, Schomp in south Denver, others give this place high praise and say it is the place to go for maintance. Plus their dealership is updated unlike the others. (This is my recommendation for you - I will be going here on my next visit)
I live about 15-20 mins from Gebhart & Murray and Schomp is about 35-40min but I do not have any confidence in the closer ones. Plus Schomp looks more like some of the other BMW dealerships I've been to in Austin and Charlotte.
Hope that helps.