This London quartet brought Americana back to the States—and to stadiums worldwide. On 2009 breakout Sigh No More—highlighted by the banjo-fueled fury of "Little Lion Man"—the band roars through an expansive blend of bluegrass, country rock and alternative folk. They've since expanded on that booming, anthemic sound, incorporating keys and horns on triumphant ballad "I Will Wait", adding smooth, bubbly grooves alongside African artists like Baaba Maal, and even abandoning the acoustics altogether for dreamy, glittering electronic number "Believe".