Take in mind the Thomas Haider Program
The best UC would be the school where you have the highest chance of getting a competitive GPA, that offers great Medical school advising and opportunities in finding Medically related EC’s. Any of the UC’s would fit this criteria. It is up to you to find the best fit, academically/financially and socially.
UC Irvine has an abysmal medicsl school acceptance rate and UCSD is below the national average- something to consider.
UC Riverside is supposed to be the best UC for premeds. It even has dedicated programs to that purpose.