There's a lot of different kinds of favourite. Octavarium's the best. When the wind's right, and it catches me off guard, I still sometimes hear Octavarium as new, and it's absolutely brilliant, but I wore it out so much ten years ago that it doesn't often taste quite the same. It's become like overchewed gum. Entirely my fault, not at all the composers', but how would that factor into a ranking? The song's the same, it's me that's changed - but should the best songs be endlessly replayable? Likewise, there are other songs that I don't think are particularly excellent, but I could listen to over and over again. Supermassive Black Hole is like that, it's not exceptional but it's a perfectly formed little pop song. I could have SMBH running on loop - and some days, in my head, I do.
So with that proviso, and the understanding that Octavarium's always my favourite favourite for legacy reasons, Illumination Theory is - currently, today, to me - the maddest, most bold and brilliant thing in Dream Theater's discography. Utterly phenomenal.