There are different criteria here:
1° purposely weedout v. natural attrition: some classes, especially at large universities that must cut the number of premeds, make it impossible for more than about 20-25% to get med school worthy grades, even if potentially 30, 40, or even 50% of the group would be "qualified"in terms of mastery and work. Natural attrition is when kids who didn’t do the work, don’t understand the materials, or figure they don’t like biology or chemistry as much as they thought they did/would, switch majors. Berkeley, UCLA, Purdue… use weedout classes. Smaller colleges have an advantage here because they seek to keep all students, as much as possible; education is much more personalized (you may picnic with your personal adviser during orientation, who may handle only 6 freshmen because your college has a 11:1 ratio, v. a large university where you don’t get a personal adviser until junior year and where advisers have to handle a a hundred students, none of whom they know.) However it means you have fewer choices for class times (there may be only 3 sections of Biology so you may have to take an 8 am class).
2° collaborative v. competitive: some universities have a competitive student body overall; others have a more collaborative student body. You can compare descriptions in Fiske for Williams and Grinnell, or WashU and Emory.
The best environment for premeds is one that is supportive, doesn’t weed, and is collaborative. However those criteria may result in a less prestigious college, which doesn’t matter if the student gets into med school but may matter if they change their mind. A good rule of thumb is whether the student is top 25% - but it shouldn’t be at a college that lacks resources, whether because it’d lower the odds of a successful application or whether (most importantly) the student will not get into med school.