This is hard to do, but I gotta be honest.
1) Tupac : For the amount of pure bangers, he put out songs about life.
2) Cunninlynguists : Knockin beats, uncompromised lyricism untainted by desire for radio play, mellow music.
3) Eminem : He's never been known for subject matter, but his lyricism and delivery are unrivaled.
4) Juvenile : The rap game wouldn't be the same without him, glad B.G. did a feature on his new album.
5) B.O.B. aka Bobby Ray : I fucks with his music all day, if you haven't heard get on it.
Biggie was dead before I was old enough to buy CDs, and doesn't motivate me to reminisce.
Jay Z ... the nigga is just boring to listen to a whole album of. I liked American Gangster tho.
Nas ... is even more boring than Jay.
Its all personal preference, really. There can be no right/wrong answers in this type of discussion, because its all opinion. Just like with sports. MJ is widely regarded as the GOAT, but I know people who will tell you that Kareem Abdul-Jabar is the GOAT. There is no way to really quantify who is the best ever at something, because there are so many different factors. But IMO, cats would kill to have the type of career Jay-Z has enjoyed. Longevity, Flow, Hit songs, big sales, classic songs/albums, big money, popularity, quotables, copycats, respect from the elders and youngins alike... the list goes on and on like E. Badu...