I think of Outkast as R&B/hip-hop/pop with R&B taking precedence. And their pop side eclipses their rap side often too especially on the later albums. Obviously they're excellent rappers as well, but for me the biggest appeal of their music is in the hooks and production.
Speakerboxxx/The Love Below is my favourite Outkast album actually! I think of it as one album. They put it out as one album and they've never reissued them separately, they're only ever referred to as a pair. So it's one album as far as I'm concerned. Of course it's damn long so I guess they couldn't expect people to sit through the whole thing in one go. So it's fair enough that they kept them separate. I can't help but feel the tracklist would be a bit stronger if the two halves were mixed together, but I still like it pretty much all the way through which is crazy considering the length. Of course it would be better if it was shorter, but it's got so many of their best songs it's ended up being my favourite anyway. If we're comparing the two discs Speakerboxxx takes the edge because it's shorter and has less filler (I also always listen to it first). But aside from the skits there isn't really that much outright filler to my ears. The genre hopping in Andre's half keeps things fresh. And Big Boi's half is just a really strong set of Outkast songs.
Not really into their debut as much. Still a good album though, they never really had a chance to put out a bad one (Idlewild comes close, but it still has some stand-out tracks).
I'd rank them almost in reverse chronological order. Speakerboxx/The Love Below > Stankonia > Aquemini > ATLiens > Southernplayalisticadillacmuzik > Idlewild
I'm sure I'm in the minority for preferring their later albums. The early ones are the ones real hip-hop fans tend to like. The later albums are the pop sell-out albums haha. But I still like 'em better.