There are locations all around Tokyo and they are all pretty good. My favorite is the tori-ten oroshi udon. Cold, thick noodles covered with a large helping of grated radish, ginger and topped with 3 large pieces of fried chicken. (You can order fried chicken breast as a topping, but the tori-ten is dark meat and much tastier).
A popular dish amongst my co-workers is the curry cheese udon. If you can't decide, just try the daily teishoku which is usually a warm bowl of udon with maguro-don or tempura-don.