I'll chime in with what has become my new favorite after a Chicago thin crust attempt the other night, and that is Polly-O brand cheese. I have never had anything that melted better and didn't get rubbery despite being pretty brown on the top. It is a higher moisture cheese I think, so you may want to only use it on an outside layer and not in a deep dish application. I tried some full fat mozzarella in a Chicago deep dish the other day and it was quite watery and made the pie a bit soggy.
Polly-O isn't the cheapest, but it isn't extremely high either. I get a 1 pound block at Wal-Mart for around $5. Definitely worth it. Good luck and let me know what you end up finding!