Asuka is my most favorite Tantanmen place in Tokyo. It's located south of Shibuya station, across the pedestrian bridge from the West exit (where the Moyai statue is located). The place is often packed and you'll have to wait your turn, but never for longer than two or three persons.
They have several different kinds of ramen - standard shoyu ramen and chashumen to tantanmen and cold ramen too (during the summer). The first time I visited, I ordered their chashu tantanmen. I immediately fell in love with their soup and noodles. I often describe the taste to my friends as very similar to Buffalo wing sauce. It's very spicy, but so addictive, you can't stop having more despite how sweaty you are getting. The chashu was also very good.
However, if you come to Asuka, you are missing out if you don't order their paiku. Paiku is fried pork ribs. The paiku-tantanmen combination is just perfect. Of course, it can't possibly healthy for you either (but if you care about your health, you shouldn't be eating ramen anyway, so you might as well go all out).
Also popular is their hiyashi, or chilled, ramen. They have hiyashi tantanmen, which I'm definitely ordering next time. I believe they only serve this during the summer.
I definitely recommend this place. I came here almost everyday for about a week when I used to work in the area. It's the best!
I must begin by telling you I have been going to Musashiya for 5 years now without growing tired of the place. I've visited tons of other ramen places across Tokyo and waited in lines for countless of hours. But I always find myself coming back to this old place. It's just perfect.
The ramen soup here is standard tonkotsu/soy sauce based. It's not too white and smelly like plain tonkotsu, but thick and heavy enough to fill you up good. Toppings are very standard: chashu (roast pork), nori, and spinach. The chashu isn't too soft and oily like some places can get. It's firm but gets soft as it soaks in the soup. The noodle is thick and short. It looks like udon noodles, but when you eat it, it's definitely ramen noodles. The noodles here are quite possibly the best I've ever had in a ramen place.
In addition, you get to choose how your ramen is done. The thickness of your soup, the oiliness of it, and the hardness of your noodle. I see others ordering extra nori or without spinach, whatever your fancy. I always order the chasumen with slightly harder noodles. For 750yen, you won't find a better bowl.
Other than ramen, there are fried rice for 100yen a bowl and a small bowl of lettuce with dressing. The ramen place next doors is also by the same people and serves many more non-ramen dishes. The sign outside mentions "the second best shogayaki (ginger pork) in Kichijoji."
Oh, and on your way out, don't forget to grab yourself a popsicle for only 30yen more.