I think the Ultrasone Edition 8 is probably the best portable headphone I have heard. However, their price tag really makes me question the "portable" part. I don't think a lot of people would throw one of those pairs around on trains and buses.
A close second, and also my personal favorite, is Audio Technica ES10. They have a very even sound signature (even more even than the midrange-shy and sub-bass-happy M50 that I have seen used in some studios), and they are relatively easy to drive out of just about anything. Only drawback is that their shiny ear cups get scratched easily.