Rakim (for the flow & science obviously)
Kool G Rap (for the mafioso)
Outkast (they're like equivalent of Prince)
I don't really want to put down anyone else. I think Dre has always appeared to be a step ahead of everyone else sonically, but I don't know, he changed the game but people were so quick to follow that it's hard to say he was ahead of his time. I think in years to come people will look back on The Roots as being ahead of their time for fucking with live instruments.