Along with Slipknot, Los Angeles-based System of a Down are far and away the most impressive mainstream metal band today. Despite the fact that they appeared amid a maelstrom of crappy Korn-inspired rap metal that was controlling the airwaves, their hyperactive song structures and truly warped vocal stylings put them way beyond the pap being served up by many of their Ozzfest colleagues at the time. When music this extreme makes it onto the radio it's always a good thing. They broke nationally with their second record, Toxicity, which shot to No. 1. Through the next several albums, System of a Down took lessons learned from Tool and other bands from the "alt metal era" of the early 1990s and transmuted them, adding eastern European folk elements, a wide array of traditionally non-metal instruments, and strong political content in the lyrics department. By the time they released Mesmerize and Hypnotize, an epic album released in two parts -- and seven months apart -- in 2005, the band had ramped up their vision, incorporating a wider range of influences than ever before, with the vituperous madman vocals of singer Serj Tankian as potent as ever.