usnews.com has a (questionable) list of the rankings of the medical schools based on research and based on primary care (separate). Other than that, its kind of pointless to separate schools based on tiers. Any top 20 school is incredible, and you can't really say that the 21st ranked med school is on another "tier" than one ranked above it.
i really dont think they put med schools into tiers. other than wanting to go for research, ranking medical schools isn't like ranking law schools. there really are not any BAD medical schools (other than, which is questionable, Caribbean schools). its mostly just your personal opinion and choices in what you find most valuable.
"tiers" matter greatly in fields such as Law, because there isn't a lot of oversight from credentialling authority (ABA), and it's fairly easy to open a school. Thus, employment opportunities to a great extent depend on your status as a grad from a "Tier 1, 2 or 3" school.
In medicine, it's another ballgame altogether. There's tight control, both with existing schools and with the overall # of schools. So you really can't say there's any "tiers" as such, because any grad from any accredited US school can in theory and practise apply for, and get, any residency.
Also, I've actually never seen any statistically valid evidence that some schools have students who consistently outperforms on the USMLE.
However, there's obviously some schools that are more famous than others. How big the group is is open for debate, but it usually includes the "household names" that your non-physician family have heard of before...