Try the MC's from Def Jux label.... Aesop Rock, El-P, Mr Lif, Cannibal Ox...

Also try:
Copywrite
Slug
Atmosphere
Cormega
Immortal Technique (my fav. underground)
Canibus (if he still counts as underground)
Talib Kweli
Diabolic
Jean Grae
Necro
Ill Bil
Cage

they all got different styles... its not like mainstream when if you like catchy hooks you will like any rapper that makes them... all these lyricists have different styles, and you wont like them all... for example i know heaps of people hate Aesop Rock or Canibus cos of the way they rap, but i love their music... if you looking for something DRASTICALLY weird and different, try Aesop Rock, Necro, Ill Bill, or Canibus