What are the strongest ballet departments?
Well that depends on what you are looking for. There are many groups (US News and World Report, Princeton Review, Forbes) that will give you rankings, but it is important to look at the indicators for the rankings. For instance the following schools get high rankings but for different reasons:
Presitige - The Julliard School, New York City
One of the oldest performing programs- Texas Women's University, Denton, Texas
Highly ranked by Choreographers - Oklahoma City University
What are you looking for? Size of school, location (urban, suburban, distance from home), commitment to major field, college values, demographics of student body, college clubs and activities available, selectivity, financial support, etc. Have you visited the school and talked with faculty and students about the program?
All of these things are important considerations when choosing an institution of study.