Any ranking system that doesn't outline its methodology is pretty much useless. The US News Rankings give a different #1 each year, and it's not because of changes at the schools, usually, but just changes in the methodology. And the rankings for undergraduate institutions aren't necessarily that helpful. They are greatly influenced by the amount of research being done, and by the grades and test scores of students entering the school, but they say very little about the actual education one will receive there. So, if you are looking to go to a school that has a lot of prestige, then the rankings are helpful. If you are looking for a school where you will have a great educational experience, the rankings are less helpful. That's not to say that the top schools don't provide great education, just that the rankings don't really show that.
The US News graduate school rankings are pretty straightforward, without any sort of complex methodology. They are simply based on the perceived quality among others in the field. I also like the rankings at phds.org, they provide a little more information about the schools, and allow you to customize your priorities.