Going to one particular UC doesn't help you get into medical school. As long as you do well and get a good GPA, you'll be a good applicant.
For example, let's say you went to Berkeley (using Berkeley because it's the oldest of all UC's) and get a 3.0, but you went to Merced (newest UC) and you got a 3.8. Well, the admissions committee will like the GPA from Merced because it's higher. Now, if you compare a Berkeley 3.8 vs Merced 3.8, admissions committee will like the 3.8 from Berkeley because they know how hard it is to do well at Berkeley.
So, I wouldn't worry too much about what school will give you a better chance. Rather, go to the school that will help you do well in school. You're still in HS. Just study and do well in HS first, then do well in college. Take it one day at a time and enjoy yourself because becoming a doctor is a very long process. Good luck! Ask more questions!