My favourite albums are IAW, SDOIT and Octavarium. All three of 'em worthy of the top spot, never quite sure who to give it to at any given time.
So it'd be one of them, paired with Dream and Day, Awake, Scenes, Train of Thought, or Systematic Chaos.
Out of those five... Awake and Scenes, while not bad albums, don't quite do it for me, Dream and Day is Dream and Day, so it was between TOT-OVM or OVM-SC.
I'm outlining my decision process simply 'cause I'm really surprised that it's Octavarium / Systematic Chaos, too. A little flabbergasted, actually, I keep looking for other combos, but no. My favourites seem to be fairly broadly split across the discography. So OVM / SC it is! Don't know why I'm so surprised, they're both superb albums. Just never realised that my top four (add FII into the mix) were so far away from one another.