Personally, I am a big fan of Ichiran in Japan just because it is usually accessible in the main tourist areas so it is a good introduction for a first time ramen introduction in Japan. I find Ichiran, Ippudo and Santouka are 3 ramen chains that are popular across Japan and all 3 have their own merits and differences. Ichiran is the most simple with just one type of broth, while Ippudo and Santouka has various kinds.

When in Japan, I have also went to other ramen shops that has unique broths though I find they might not be consistent. For example there was this ramen store I visited in Yokohama where I enjoyed the ramen but when I re-visited the place a year later, I found the broth too salty. On the other hand, my visit to the major ramen chains in Japan are usually good with no surprises.