My favorites are Radio Los Santos, Radio X, and SF-UR. The SF-UR DJ makes me laugh for some reason, the Snoop & Dre stuff on RLS is great, and when I was in high school (~10 yrs ago) I could've made the entire Radio X tracklist from my own CD collection. Even that godawful Danzig song...
And I liked the dynamic nature of WCTR. Some of the stuff gets a bit old, but I liked the bits that changed with the storyline: mission-related news, Big Smoke & OG Loc stuff. Now that I'm done with the storyline I don't listen to it much though.
At first I wasn't very impressed with the stations in SA, they just didn't have the punch (or novelty?) of the 80's stuff from VC. But I gotta say the music in SA seem to be aging well, and I'm not tired of any of them yet. By this point in VC, I was down to only a few stations that I'd still listen to...