Tupac. Biggie had the more fashionable lines, and he knew how to size up a beat better. He had more "mojo" to his songs overall, but Tupac's music was life changing though. He was a lot more symbolic, philosophical, and spiritual in his raps, and the more historic of the two.
Tupac appealed to a lot of people, had had mad amounts of charisma. The guys wanted to be like him and ladies just wanted a guy like him. That being said his lyrics waiver from mostly just ok to moments of greatness. Biggie was far superior IMHO.