With their debut album, 1991's Blue Lines, Massive Attack pretty much invented trip-hop, but they didn't stop there. From their soundtrack work to the sumptuous ambient pop of their '00s albums, the Bristol sound-system veterans have never stopped innovating. When it comes to collaborators, their picks—like Martina Topley-Bird, Horace Andy, Tricky, and reggae mastermind Mad Professor, who dubbed them out on the groundbreaking 1995 album No Protection—have pushed the limits of their sound even further.