I'll be straight up, I may be a bit biased, but in my eyes ATLiens is the perfect example of a rap album that flows together extraordinarily well, while also not having one weak track the whole way through. I'll argue that, lyrically, this just might be the greatest rap album ever made.
I can't decide which is the true Outkast classic, Aquemini or ATLiens. They're both aided by smooth, weird instrumentals, both scrawled with lyrics that are futuristic, braggadocios, and street-conscious all at once, and both are just built to perfection as albums. It's getting harder and harder to pick between the two because they're both so perfect, so we might as well make them both Outkast classics.