Well finding the "best" of anything is somewhat debatable but yet you can find part of the "best" colleges and universites in the states in second degree ecnomics:
http://www.graduateshotline.com/study/economics.html
http://econ-www.mit.edu/graduate/index.htm - was ranked as the best...
in this link you will find that Harvard, MIT and Stanford are the US best and ranked top 10 at the world.
Yet I know that there are many other.
THIS IS THE BEST LINK OF INFORMATION:
It holds rankings according to the specific fields in graduate studies:
http://www.econphd.net/rankings.htm
click on the microeconomic theory - you will find that T.A university is number 2...
Good luck...