Moulton bicycles have seen continuous development since their launch in the '60's, and the Bridgestone model takes it to new levels. It comes apart for easy packing, but its main virtue is that it's equally at home nipping through the city as it is on all day rides. The suspension means that the small wheels feel smooth, and the gearing gives you plenty of scope for being speedy. Removable pedals make it into a neat package when folded, and give an extra twist to theftproofing if you ever have to leave it locked up outside.
Also available in a non-separating version, called the Sport.