There's many different ways to rank graduate programs, each leading to somewhat different conclusions. One recent study of political science programs sought to modify the common approach (where faculty publish), in part to include more European scholarly journals. The result is a list of what they term the "Global Top 200 Political Science Departments"as well as the "Rolling Global Top 50". It's worth a read if you are considering going on to further study in political science, here or abroad. You can find the article here.