I chose the very basic I could find: a single speed Dahon Boardwalk. It has a steel frame so it's not that light, but I'm OK with the weight. The bike is simple so there's not much to break and for a year and a half of (ab)use it has been solid. Where I live it's mostly flat, so it may not be the best choice for hilly SF for example, although I hear fixies are popular there. Bike forums are best to discuss these matters.