The Pittsburgh rapper transitioned from teen phenomenon to serious musician with a series of genre-hopping guest appearances. On Earl Sweatshirt's low-key "Guild", MAC drops gruff, densely written bars, yet he's equally adroit at bantering with Ariana Grande on the '90s homage "The Way". His chameleonic style is a slick match for any track, adding a rapid but eloquent flow to hip-hop and pop.