The Smoker Friendly on Parker and 225 used to have a decent selection. They've stopped carrying a lot of the boutique stuff.
Stogies and Bogies has a decent selection in Lone Tree but of all the places I've visited in South Denver, Smoking Cave has the widest selection in highlands ranch.
Cigars on 6th is good if you're near downtown and Havana Manor is what I recommend near the Boulder Westminster area.