Getting 2Pac on your track was always a two-sided coin. He'd bring the lyrical ruckus, but who's going to remember your verse? Fortunately, tracks like “Runnin' (Dying to Live)” with Biggie and “Thugz Mansion (N.Y.)” with Nas and J. Phoenix are rap versions of a superfight: top heavyweights trading blows in the middle of a ring. Check out some of 2Pac's greatest collaborations.