Hey - I never answered the thread question!
I voted Octavarium, but also realised I'd never particularly thought about it. It's almost apples and oranges, for me. Octavarium is a single, clear journey - an A to B epic. While I respect that the band composed Six Degrees as a single song, they also decided to index it separately so you could listen to the individual tracks on their own, so it's sort of up to the listener. It's my head, it's in a weird space where it's either quite a long, scattergun song, or it's a super-cohesive album, and I tend to prefer the latter. The latter makes more sense to me. Makes it more than an album, instead of less than a song.
Basically, I don't know what Six Degrees is - but I think, if you had to tease an answer out of me, I'd say it's whatever The Incident is! The Incident's the only thing I can think of that's even slightly similar, but it's very similar, right up to the anthology of unrelated tracks on the opposite disc. That's good, though. New forms of musical storytelling are good, they're to be encouraged, and I love that DT beat Steven Wilson to the punch on this one, but it never really occurs to me to pop SDoIT in the same bracket as A Change of Seasons, Octavarium or Illumination Theory.