I think all of those factors are big considerations when selecting a medical school. I don't think one could just use one of those categories to select a medical school.
With that said...If I had to choose one category that was most important to me it would be location, location, location. A lot like real estate. 😉 I'd say the next most important thing if not as important as location is the school's ability to place its students into good residencies. Then I'd look for curriculum and grading system. Ideally I'd want to go to a school that is pretty good in those three main areas.
I don't think other factors are as important if important at all. I think EVERY school will have cool people and annoying people. Every school will have good and bad professors. For the most part, every single school I've visted has happy students. Its funny because I read all these interview feedback reviews and for the most part students are "genuinely happy" at their school. hehe. I'd say yeah they are happy, especially if its a top 30 school, because quite a few kids in those classes had the opportunity to choose their school over another. Of course they are going to choose the place they would thrive the best in. Cost can be a big issue depending on the individual. For nontrads its probably a big factor. For trads my take is that yes it seems like enormous debt. But it is a debt that can and will be paid off as a physician. I don't think costs should negate the big three considerations. Certainly the degree of its importance is dependent on the individual.
Of course there are other issues at stake...having children, a wife, etc. Those factors would automatically be as important if not more than the big three.
just my 2 pennies.