Definitely Tupac. Easy win.
Em was great in his prime, circa Slim Shady LP and Marshall Mathers LP, but 'Pac was more than just a good rapper. His lyrics and delivery still give you chills 13 years after his death, that's how good he was.
The harder question to answer is, who is the better SINGER? Hahaha, neither one can sing worth shit, love Pac to death but he couldn't sing. Neither can Em.