What universities have strong Economics departments, strong specifically in economic theory?
Off the top of my head, MIT, University of Chicago, Harvard, Princeton, and Stanford are some of the better-known schools for Economics in the states. Of course there’s the London School of Economics and Cambridge in Britain.
As far as Economic theory, I’d like to say U of C, (1) Because as a rule the University is interested in theory over application, and (2) I’m going there next year 
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Do you mean “economic theory” as in Arrow’s theorem, the Gibbard-Satterthwaite theorem, cake-cutting algorithms, and other mathematical meat? Or do you mean “economic theory” as political science mastu^H^H^H^H^Hideas?
Hey, good luck! My mom got her undergrad from Chicago in Econ and her Masters at USC. She’s now worth millions.
The George Mason University Department of Economics is highly regarded.
What Splanky said.