I'm a first year grad student at UCSC trying to study theoretical cosmology, so I can give you my very biased opinion. Keep in mind that I'm kind of in my own little bubble as a first year, and I lack both a reasonable basis for comparison with other schools and the broader perspective of my own school that, say, a fifth-year might have.
That said, I think we have a really good cosmology group here. Joel Primack seems to be kind of a Big Shot. If you're willing to sit and listen, he'll be more than happy to tell you about how he was intimately involved in at least half of the ground-breaking discoveries in cosmology in the last 30 years. He has an absurdly high number of students that later go off and get faculty positions (something like 90%). He mostly does physical cosmology though, along with a bunch of people in the astro department (Piero Madau probably being the most prominent) and a few other physicists. On the more early universe side, there's a small group of theorists in the physics department that seems to be quite strong. Anthony Aguirre works on things like eternal inflation and colliding universes, Stefano Profumo works on the nature of dark matter, and Michael Dine seems to do a variety of stuff on the border of cosmology and high energy theory. There's also Tom Banks, but he seems to be more of a pure high-energy theorist instead of a cosmologist. I suppose it doesn't seem like that big of a group when you count it out, but then again theoretical early-universe cosmology is a pretty niche subject. You should check out the TASC webpage for more of a broad overview of theoretical astrophysics here.
I think that one of the things that Santa Cruz really has going for it is that all of the professors here are very friendly and approachable, and I haven't met anyone here that I wouldn't want to work with based on a personality conflict. Of course, your impression may be different.
Hope that helps.