Nagi - BUTAO Shibuya
The Nagi ramen chain has several locations scatted around Tokyo, the most well-known of which is in Shinjuku's famed bar district, Golden Gai, up a steep, cramped staircase. Its adherents believe it to be the leading chain proprietor of ramen in the city. This location, in Shibuya, is in a more quiet and residential area, with more spacious table seating.
The ramen here is pork-based and rich, with the ability to customize the order heavily for toppings and noodle preferences. The most popular options are Butao, Huroo, Akao, Midorio, Sansyoo, Kosyoo, Ebioo and Sakanao; probably the safest choice for one-time visitors is the butao with added pork belly and medium noodle thickness and chewiness. The quality of the ingredients and execution is extremely high, and it's hard to find fault with the cuisine.
Although markedly less atmospheric than the Golden Gai location, in my experience many people prefer tonkatsu-based ramen to noboshi ramen, and if that's true of you, you may want to prioritize the Shibuya location, particularly if you have any sort of aversion to cramped quarters.