It's like just off the main road, in a little alley. The place opens at 10am till ultra late night. They're very friendly (to us who can't speak Japanese), and it's a self-serve system. You tell the cook what you want (including which version of udon), he doles out the bowl of udon to you, you proceed to the yakitori section (if you want to add any sticks or pieces of fried stuff to your bowl), then you move on to the cashier. You can take plain water from a separate dispenser for free.
Fr Shinjuku station west exit, cross the road to halc odakyu, turn left and walk down the street. About 5-10min, u'll pass a cross junction, several convenient stores. there'll be a y-forked road on the left, with a eatery at a corner. turn into the upper fork, mentsu dan is one min on your left.
Favorite Dish: I had niku (beef) udon's - it was fabulous, more than the rest of the niku udon i had elsewhere in tokyo.
By far the best udon in Tokyo. Forget fancy restaurants, this place serves fresh home made noodle and you can choose different kinds of tempura, natto and fresh egg yolk. You pay extra for every item you add.
Favorite Dish: Hot udon with egg yolk and natto