Long time lurker here. Just decided to open an account to give my two cents regarding disoriented's question. Keep in mind that, since I am interested in string/field theory, my comments will only focus on those fields. First of all, you have excellent options to choose from. From a purely academic point of view, I would put OSU at the bottom of the list. Now, with the other three, I think it depends on what subjects you are more interested in. From your opening post, I take it that you are not sure yet, so, it probably would be better for you to go to a place with a large group and broad research interests. Now, the "problem" for you is that UCB, UCSB and Chicago fulfill these requirements!
These universities have very strong groups in string/field theory. Let me give you a list of professors at each of these places that are very well-known and active experts in the field; maybe you can make up your mind by browsing their websites. In no particular order:
- UCSB: Polchinski, Horowitz, Giddings, Marolf, Berenstein, Morrison and Gukov.
- UCB: Aganagic, Horava, Bousso and Ganor.
- Chicago: Kutasov, Martinec, Harvey and Sethi.
Personally, I would choose UCSB for several reasons: the research interests within the group cover almost the whole spectrum of current research in string theory, the location, more supervisors to choose from, etc.
Again, remember that all I wrote was focused in string/field theory!
Hope this helps