Who else but rap’s most fearlessly creative duo could open their biggest album with a bonfire of American dreams? Outkast’s fourth LP may have launched them into the mainstream, but there’s nothing safe about it: Even its most populist track, “Ms. Jackson,” is a glitchy, galactic soul number about a messy divorce. The collection is peppered with slo-mo funk cuts (“Toilet Tisha”), neo-soul steppin’ anthems (“Humble Mumble”), and political drum ’n’ bass rap (“B.O.B.”). There is no genre, no “mainstream versus underground”—there is only groove.