Kool Keith
Redman
Aesop Rock
MF DOOM
Those 4 are pretty much etched in stone on my list, the 5th spot could go to any number of guys.
Gift of Gab
Lord Finesse
Del
Aceyalone
Masta Ace
Rakim
Guru
Ghost, Gza, Deck, Meth
El-P
Sean P
Pharaoh Monch
Sadat X
Eric Sermon