Since its beginnings, hip-hop's most explosive moments have often been its collaborations between the genre's biggest stars. When two rappers team up—unexpected or not—the results are compelling in a way no other genre has ever captured. Rappers know this: grabbing a feature from a big name artist can make an up-and-comer's career, and when A-list stars decide to hop in the booth together, the results are attention-grabbing even when they don't quite work. And when they do work, the best thing to do is just keep getting in that studio together. When artists collaborate frequently they develop a kind of alchemy—everything they're on together becomes more than the sum of its parts—which is why when you see, say Drake and Future next to each other on a tracklist, you know it's going to result in something special. It's those connections—the rappers who keep working on each other—that make rap such a dynamic, versatile genre. These are the best unofficial duos (and one trio) that produce nothing but the best when together.