Barter 6 was billed first as Young Thug’s debut album, then a retail mixtape; either way, the 2015 release was the sharpest, clearest statement yet from Atlanta’s most enigmatic rapper. From the gently rippling intro, “Constantly Hating,” to the swirling, cathartic haze of “Just Might Be,” Thug contorts his voice into endless shapes and pulls previously unheard harmonies out of his back pocket. “Check” is a giddy celebration of success, and “Halftime” is rap as high-wire routine: a technical performance as reckless as it is graceful.