It all starts with the beat. Rappers are becoming stars more quickly than ever, and which producer they choose to work with is probably the most important decision they will make. Obviously, there are producers whose presence on a track will automatically make you sit up and take a listen. But even more interesting and vital are the beatmakers who are next up. These producers aren't household names yet, but each one has shown the ability to jumpstart an artist's whole career. You should be paying attention.