What are the best universities for studying computational social science?

It depends a great deal on what your definition of CSS is. For a lot of people, CSS deals with methods like machine learning, data mining, social network analysis, and other things that people commonly associate with "data science" per se. For others, CSS has more to do with social simulation modeling. There are people (like a few of my George Mason classmates) that do a mixture of both, but otherwise most people lean way or another in terms of their interests. I would think a bit about about that question and then come back.