I would actually have to say High Grade Alternatives on Arapahoe. They have good meds and hook the bags up nicely. I went there on Thursday when they have certain strains for $45 and hooked up my bag of Maui to 3.9g and the $45 included tax. The front desk lady is super cool and you can put in an order and pick it up if you are in a hurry. There isn't much difference between their 'A' shelf and 'B' shelf and they have around 20+ strains of each kind.
Grateful Meds is a classic place in a classic town. I feel like some quality has been lost but they keep it real. 40+ strains and fat bags.
Stone Mountain Wellness is a good place to but Scott, the owner, sold out to become a a commercial growroom operator and some rich people from Denver bought the place and has plans to make Stone Mntn a 'brand-name'. They are in Longmont and they have some of the best money can buy but I'm not too sure for how much longer. They are nice people with a specialty in Kush strains.
Now, the best bud in Boulder.....
Botanic Labs in Longmont has some of the best bud I have seen and its consistent. I've been dealing with Matt ever since I got my card and his strains still get the best o me from time to time. Lifesaver is a great painkilling bud and he has some that just glistens with the light. The only problem about BL is he charges $20 a gram if you aren't his patient but its definitely worth the money.