Cas I nice but I just can't vibe with him for some reason. But while we are talking best lyricists, not best rappers, I think Papoose deserves a mention. Only mainstream song I've heard him on was Busta's Touch It remix and he murdered that.

Papoose - Take The Blame 2012 (Hot Fast Freestyle) (HD) - Whiteowl - YouTube

Papoose - Alphabetical Slaughter - YouTube