Nakamoto is not your typical ramen restaurant, but it does seem to be one of the more well-known and is the first place anyone mentions if the conversation turns towards spicy ramen and has a number of branches around the major hubs of town. This one is in Okachimachi is one of a cluster of ramen shops under the Yamanote line tracks.
The soup is tanmen, which is a more traditional type of noodles from China that comes with stir-fried meat and vegetables, but the real fire comes from the Mapo Doufu, a dish from the Sichuan province of China which famed for being the home of all sorts of spicy creations.
It was definitely the spiciest ramen I've had, and alarmingly was only a 4/10 level of spiciness according to their menu. You'll definitely want to keep one hand on your water while you're slurping away.