I think a "true roadbike" would not be so fast in most citys, apart from maybe Copenhagen.. You need a fairy flat surface, no pot holes, no glass, curbs and so on. They work on the roads around here and more than once I`ve seen peopel carry they road bikes becouse of some small stones spread out on the tarmack. That is not fast. Some places the city senter is in such a shape that a MTB w front suspension and cushy tires is the way to go.
The best bike is the one best suited for the riding where YOU ride, and the one that is still there and easely available the next time you want to use it. Maybe becouse it was not stolen for the way it looks, maybe becouse you folded it and took it innside with you to avoid theft.