Tupac's cool and everything, but I think he's veeeeery far from being an important person in history. Same goes for Bob Marley and John Lennon. These people are the voices of movements, the spokespersons. However, they don't start the movement, they just give it a voice.

Like it or not, the most influential person, IMHO of the 20th century probably is Hitler. Influential doesn't mean it has to be in a good way.... He impacted the entire planet and directly or indirectly caused more change than anyone else in that century.