Depends on what you like. Permutation City and Diaspora are fan favorites. They are heavy on idea porn but not very good yarns. That goes for most of Egan's books. His short stories are probably his most polished works because the form's constraints force him to keep everything tight. But on the flip side it means he doesn't have time to develop his ideas in great depth.

If you're a programmer, Permutation City would be my recommendation. If you're more mathematically inclined, Diaspora is a good choice. Just so you know what kind of book you'd be in for, the first chapter of Diaspora has a proof outline of the Gauss-Bonnet theorem. His short story The Planck Dive gives a good flavor of Diaspora: http://gregegan.customer.netspace.net.au/PLANCK/Complete/Pla...