You want to find the school where YOU will get the maximum quality instruction.
Things to consider...
1) Who is teaching the class - in particular, the lower division classes. Are you getting a grad student whose command of the English language is suspect? Some schools do a better job of this than others.
2) The "star" student generally gets better professor access. While you can get some amazing professors at the top schools, the competition for opportunities can be quite intense. Sometimes more attention from a professor at a lesser school (for undergraduate work at least) is better than less attention from a professor at a greater school.
3) Do you have a particular interest in a specialized area of math, or have you explored the various specialties? This gets down to picking a school for a particular professor or two if you have an area that has your dedicated interest. Most high school students haven't had enough exposure to make that decision, but I had to ask.
If you cannot answer these questions, I go with S&H's answer with a caveat of affordability. Go where you would even if Uncle Sam isn't paying the tuition, just in case that ends up being the case. Stuff happens.