Once again, I would like to point out that there's no such thing as a "best uni for Science", as Science is a
very big thing with lots of different fields, each of which is stronger or weaker at the various universities. First decide which field you want, then look at the programs and see which faculty best fits your needs. Medical Science specifically is a pretty standard degree, with extremely similar course content across the big three schools (Macquarie doesn't count because they don't have any resources, their third year students are sent to USyd). For MedSc I'd suggest looking at which university offers things beside course content that are particularly attractive to you, such as student facilities, ease of transport, flexibility of the course and even social things like whether your friends are likely to go there. I know that in six hours at USyd I see more people from my cohort than I would in a week at UTS, and I couldn't stand that for three to five years.
Good luck in deciding.