What are some popular ramen chains in Japan?

Japan is home to countless ramen chains, each with its own unique style and flavor. Some of the most popular chains include Ichiran, which is known for its customizable tonkotsu ramen and its individual 'flavor concentration' booths; Ippudo, which is another famous tonkotsu ramen chain that has expanded globally; and Ramen Jiro, which is a cult favorite that is known for its massive portions and its rich and fatty broth. Other popular chains include Santouka, which is known for its shio ramen, and Menya Musashi, which is known for its innovative and creative ramen.