Jay, Em, & Nas are my Top 3.

Then in no particular order would be Biggie, Jadakiss, Tupac, Meth, Red, Styles, Big L, Outkast, Dre, Snoop, Game, rest of Wu, Cube, Talib Kweli, Common, Pun, AZ, KRS, Luda, among a few others.

And to me there is absolutely no difference between 'underground' and 'mainstream'...it's all music, but if you insist...Andre Nickatina, Akrobatik, Mr. Lif, Dead Prez.