The NCAR building in Boulder is definitely one of the best brutalist buildings around. The coloring helps a ton, and its context alone atop a mesa with spiky mountains as a backdrop works artistically in a way urban buildings just don't. I hate to admit it, but I kind of like it.
Boulder's other famous brutalist building is the engineering center at the University of Colorado:
source
CU's campus architectural plan has required the use of those red flagstones for decades. The result is a cohesive and distinctive campus aesthetic that is widely regarded as one of America's most beautiful. It doesn't translate well to brutalism.
... I wouldn't bother bringing it up, but in the background of that middle picture you can see Boulder's third most famous example of brutalism. Those yellowish high rises in the distance are part of Williams Village, an off-campus cluster of dorms for the university.
Since they're off campus they aren't beholden to the flagstone requirement. They are the tallest buildings in Boulder, and along with one other residential highrise built about the same time are responsible for Boulder's strict 45-foot height limit. Their ugliness was such that Boulder residents rose up and demanded the city never again be so sullied.
I lived in this complex for one semester. The large windows on the top floor are a study hall. On particularly windy days (of which Boulder has many) you could literally feel the building sway from up there.
source
source