No particular order (this is just my favourite, so that's why they're in my top five)
- Masta Ace
- Tonedeff (check out Love Aint, my fav)
- Cunninlynguists (the group as a whole)
- Immortal Technique
- MF Doom
- Common (phenominal)
And Mike Shinoda shouldn't get a mention in this thread. First of all, he's not underground. His songs are on the radio all the time. Second of all, the only thing that makes him worth listening to is his beats, and some of the guys he works with. His rhymes are too simplistic.