If 2Pac counts as old school (I've never really tought of him as an "Old School Rapper"), then definately him.

If we're talking about rapper from the 80s and early 90s, then I'd have to say KRS-One, Rakim, Slick Rick, All of NWA, and a whole bunch of others that slip my mind right now.

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"With all this extra stressin
The question I wonder is after death, after my last breath
When will I finally get to rest" - Tupac Amaru Shakur, "Me Against The World"