/u/op00to: This is challenging because ramen is classified by a mix of base, seasoning and thickness.
Roughly bases can be: bones of chicken, pork, fish or in rare instances beef. Often there is blend of these bases
Seasoning: sea salt (shio), soy sauce (shoyu) and miso
Thickness is a spectrum ending with: kotteri (thick) and assari (light)
Now, how to pick a light broth? This will be highly context dependent. Most shops in Tokyo where I'm living right now are specialists so research will show you upfront whether it's tonkotsu (heavy pork), chicken paitan (heavy chicken), hakodate (light fish), tonkotsu-gyokai (blend of pork and fish).
If you're in Japan, I recommend the Ramen Beast app which has location based recommendations, high quality reviews and lists for lighter ramen.
If you're in NY and looking for a lighter bowl: I can vouch for most of the bowls at Nakamura, the yuzu shoyu at Mr. Taka, and all of the bowls at Yuji Ramen.
Note that as you mentioned since most shops in Japan specialize in a specific type of soup (i.e. the only differentiator is the tare and the toppings), it might be more useful to familiarize oneself with the characteristics some of the more common regional styles. For example, "Hakata-style" ramen generally describes a creamy tonkotsu soup, "Kitakata-style" is generally a chintan with shoyu tare and hand kneaded "temomi" noodles, Yokohama "iekei" generally tends to be a paitan with shoyu tare, Sapporo ramen is generally a pork soup with miso tare, etc.
This is not to say that e.g. Sapporo only has Sapporo-style ramen or Fukuoka only has Hakata-style ramen, but the names describe where the style is perceived to have originated from.
The water is slightly muddied (no pun intended...) by techniques like "double soups", i.e. mixing multiple soups in the same bowl. In some cases a chintan might be mixed with a paitan, etc.
Seasoning: sea salt (shio), soy sauce (shoyu) and miso
Thickness is a spectrum ending with: kotteri (thick) and assari (light)
Now, how to pick a light broth? This will be highly context dependent. Most shops in Tokyo where I'm living right now are specialists so research will show you upfront whether it's tonkotsu (heavy pork), chicken paitan (heavy chicken), hakodate (light fish), tonkotsu-gyokai (blend of pork and fish).
If you're in Japan, I recommend the Ramen Beast app which has location based recommendations, high quality reviews and lists for lighter ramen.
If you're in NY and looking for a lighter bowl: I can vouch for most of the bowls at Nakamura, the yuzu shoyu at Mr. Taka, and all of the bowls at Yuji Ramen.