It depends where you live.
I live in the Bay Area and I'll tell you Verizon is the best here. T-Mobile is not bad (I use it), but Cingular really sucks compared to the other 2. If you live in the dorms like Foothill or Clark Kerr, you will get CRAP reception with CIngular. My roommates had to leave room to talk with Cingular. T-Mobile is not that that great, and I have to stand by the windows (hey at least I didn't have to leave the room), but Verizon will do fine. As for the southside dorms like Units 1 - 3, you will be fine with any carrier.
Reception with T-Mobile is awesome on campus. Once you're on you get like 5 full bars and it's like WOW. I've never had a problem with reception in SF with Sprint or T Mobile.