If you are hunting for a terrific graduate-level text in microeconomic theory, pick up Varian's 3rd Ed. Microeconomic Analysis. The book is rigorous, but not at all overwhelming, and is replete with the kinds of exciting results that made you major in economics as an undergraduate. Moreover, the book is concise -- the author seems to recognize that taking a long time to explain relatively straightforward concepts is not a way to endear onesself to one's readers. (Unlike a certain very large and cumbersome graduate microeconomics textbooks that came out recently.)
I saw some comments about the book requiring topology. I beg to differ -- while the math requirements are nontrivial, they are not so severe as topology. Anyone with a good background in linear algebra and multivariate calculus will find the book approachable. A course in rigor and proof, such as "Foundations," might be useful, but one can pick up such details from Varian's text itself. Differential equations and Real Analysis help, but are by no means essential.
This is a terrific text for graduate and highly advanced undergraduate economics students. However, I suspect many graduate business students will find the mathematics unweildy and perhaps a bit unnecessary for their purposes.