The Florida-born songwriter Matthew Musto helped define the underground SoundCloud rap aesthetic, but he's equally comfortable at the top of the charts. He co-wrote "Boyfriend" for Justin Bieber, merging R&B swagger with an acoustic riff, and brought a mournful sheen to Linkin Park's "Sorry for Now." As an artist, he's unapologetically introspective but never one to sulk, always ready to cry at the party on sensitive bangers like "i miss the old u."