I use to listen to what I thought was underground hip hop: mos def, kweli, the roots, common, cunninlynguists, zion i, aesop, del, dilated peoples, hilltop hoods, MF Doom etc.
And then I realized that like half these people are extremley well known so I stopped calling them underground artists. Then I realized that being well known doesn't mean your a bad rapper. I still don't listen to people on the radio like dj khaled, lil wayne, ti, young jeezy, etc. because I personally think they suck. But theres a lot of artists that are well known and are still great. Like nas, biggie, jay z, tupac, even kanye, wu tang, mobb deep etc.
I definitly listen to what you call 'underground hip hop' but honestly thats a terrible name because theres so many people that think they are hot shit (I know I'm guilty of the same but not like them) because you don't know about the artists they listen too.
Honestly just listen to what you like and stop making the distinction between underground and well known because there is none, its just hip hop. Who the fuck cares how well known someone is. Most real underground artists suck or else they would be somewhat well known