The short answer is: MIT, CMU, and Stanford are historically the
powerhouses of AI and still are the top 3 today.

There are however, hundreds of schools all over the world with at
least one or two active researchers doing interesting work in AI.
What is most important in graduate school is finding an advisor who is
doing something YOU are interested in.  Read about what's going on in
the field and then identify the the people in the field that are doing
that research you find most interesting.  If a professor and his
students are publishing frequently, then that should be a place to
consider.