The only schools I can imagine as not being "respectable" are the new medical schools that haven't graduated anyone yet. But even for those schools, they had to meet very strict guidelines before they were able to admit their first class, so I have no doubt that those schools will be (or already are) fine institutions.
In short, all US medical schools are respectable.
edit: As for cut offs for top schools, some people consider a school's MCAT average of 35+ to be top tier (roughly top 20-25) and is probably the most common opinion, for others it's strictly the top 10, and then for others still, top tier only includes Harvard, Hopkins, and Penn -- any other school mentioned in the same sentence as these three is offensive to them for some reason