Okay, I'll preface this by saying that while I'm a CA resident, I did not attend a UC, although I did take summer session at UCSC and UCI.
If you had to rank the UC's, I would say that Berkeley, UCLA, UCSD are the top three. UCR would probably be at the bottom. UCD, UCSC, UCSB, and UCI are pretty much in the middle, with not a lot of substantial difference between them in my opinion.
Going to a school that is affiliated with a med center will make it easier to find research opportunities as an undergrad. However, you can still do very well without being affiliated with a med center -- don't forget about all the people from small, elite liberal arts colleges who get into med school each year. Their schools clearly aren't affiliated with med centers.
I had a very good friend at UCSB, who really enjoyed her time there overall, but I think the social aspect of the school can sort of predominate over other things. If you're not really into the Southern Cal party scene, it may not be the place for you.
I LOVED UCSC!!!!!!! Like I said, I only took summer classes there, but I loved that school, convinced a friend to transfer there, and if I was doing the college thing all over again, I would have very seriously considered foregoing my private school in favor of UCSC. The science programs there are excellent, the profs are excellent, and seem to actually make an effort to get to know you and take you seriously. The students were amazing -- some of the most friendly, nice, interesting, and intelligent people I've been around. I was just amazed how friendly everyone was, which counts for a lot when you are talking about premed coursework. I just loved the atmosphere there. And the setting is beautiful -- ocean views, redwood forests. I just can't say enough positive things about that school.
I know you didn't ask about UCI, but my experience left a lot to be desired. Some of the professors were just complete jerks, and it's known as a commuter campus. I didn't get too much of a feel for the students because the majority of my classmates in the summer session were from UCLA, and maybe it was just a bad group, but they were among the most unfriendly people I've ever met in my life. They made it painfully obvious why premeds have such a bad reputation.
As for Berkeley, I think academically it has a lot to offer. My main gripe about it (based on the experiences of several good friends who went there) is the size -- I think it can create a very competitive, cut-throat environment, and can also lead to you feeling lost in the cracks of the system. Personally, if I was deciding between the two schools, I think I would have gotten a better education at UCSC, just because the size of the school makes it seem easier to get to know your profs, the people in your major, etc. It just seemed like a more positive environment to work in. Again, this is just a personal preference -- I don't think the environment of Berkeley would have been right for me. Berkeley has a lot to offer, but I think you have to be very proactive to take advantage of it, and not to feel like just a number in the system.
UC-Davis -- again, some very close friends went there and absolutely loved it. The big drawback for me would be the location, or lack thereof. I would prefer to be closer to the beach and the mountains.
Anyways, try to go on some trips to these schools, look around, and talk to the students, as many as you can. Most of all, go with where you think you will be happy and comfortable -- that can play a huge role in your success during college. Good luck.
