Tonkotsu (pork bone based) ramen is good, but I'd recommend branching out and trying other ramen if possible. For example, I feel Menya Ultra in SD is pretty close to the Tonkotsu listed below.

• Tonkotsu (if you have to):
  • Ippudo (everywhere in Japan, has shops in NYC, SF, LA)
  • Ichiran (everywhere in Japan, has shop in NYC)
• Shio ramen (salt seasoned)
  • Afuri (Yuzu seasoned clear broth, Ebisu)
  • 麺屋ひょっとこ 交通会館店 (yuzu clear broth, yurakucho) tabelog
• Shoyu ramen (soy seasoned)
  • 人類みな麺類 (fish based, osaka; get the macro) tabelog
• Tori Paitan (rich creamy chicken broth)
  • Kagari (Ginza)
• Unique
  • Kitchen Troquet (tomato based arrabbiata ramen)
• Tsukemen (dipping ramen)
  • Tomita (Matsudo), need to go early morning to get a ticket, then go back later for ticketed time (probably afternoon)
  • Rokurinsha (multiple locations, including Haneda and Sky Tree)
  • Fuunji (Yoyogi)
  • Mouko Tanmen Nakamoto (Shinjuku)
• Abura soba (Dry ramen)