Depends on what type of ramen you want to eat. If you want Miso, it has to be Sumire. If you want Tonkotsu, I like Mutekiya. For Iekei, you need to go to Yoshimuraya. For Shouyu, Shinasobaya is great.
If you go to the Shin-Yokohama Ramen Museum you can hit 3-4 of the best in the country right...