You'll probably get more bang for your buck in Fort Collins. NB is a great tour (even if you don't like their beers all that much). There's also Odells, Fort Collins, Funkwerks/Crooked Stave plus some other very small places. Those three breweries are on the same road only minutes apart.
Boulder definitely has some good options. I think you'll drive more and end up spending less time at the breweries. Denver also has a ton of good options for you.
Pick whatever brewery you want to visit the most and you'll find plenty of great places in that town to visit once you're done with your top choice.