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!