Man I feel really old now because most people on here are talking about Old School and throwing out names that are 5-10 years old...that is NOT OLD SCHOOL! Run DMC, LL Cool J, Sugar Hill Gang, Grandmaster Flash,Kool Moe Dee, D-Nice, KRS-ONE and all of Boogie Down Productions, Fat Boys, Eric B and Rakim, Big Daddy Kane, Whoodini, NWA,Ice T, Kurtis Blow, Slick Rick,Too $hort,Ice Cube I could go on but THOSE are true Old Schoolers, the ones that help put rap music on the map. Expecially Run DMC, you can't mess with the Kings of Rock.
As far as newer and current stuff 2Pac owns any rapper in the last 10 years with Big being a close second. After that it gets kinda mediocre in comparison but still Jay-Z, DMX, Ludacris (funniest rapper alive) some of Ja Rule, Eminem, Scarface, Mack 10, Bone, Cypress Hill, Snopp Dogg of course, SOME of Nellys stuff, some P Diddy/Bad Boys stuff Busta Rhymes and Outkast just for pure originality of his newer stuff.
Damn I am getting old.