I had to think about this one. Like most people, my list changes sometimes depending on my mood. But I don't imagine my #1 ever changing.
1. OCTAVARIUM (I have been a fan since 1992, and I consider 8V to be the single best representation of what Dream Theater is on a single album. Other than the lyrics for The Answer Lies Within being a bit, "kitchy", it's a PERFECT DT album.
2. Scenes From a Memory (Nearly 8V's equal, but not quite)
3. Images and Words (it was my first love, I couldn't possibly place it lower)
4. Awake (the last album that I felt like I was the only fan in the world LOL)
5. Train of Thought (MP wanted to do a metal masterpiece that was like Master of Puppets or Number of the Beast....I think they achieved their goal)
6. Six Degrees of Inner Turbulence - (The title track and The Glass Prison are masterpieces....the rest is just OK)
7. The Astonishing - (Unlike most people, I think it should have been even bigger. Too many ideas seem not finished. But the most ballsy thing they've done in a long time)
8. DT 12 - (I give TA the slight edge for being more ambitious, but this is the most cohesive album of the Genie era)
9. A Dramatic Turn of Events (an excellent statement after the loss of MP....but slightly derivative, and playing it a bit too safe)
10. Falling Into Infinity (still a good album, but Lines in the Sand (their weakest mini-epic) brings it down some. It would have been better as a double album as originally hoped)
11. When Dream and Day Unite (this would be much higher if not for the horrible production)
12. Black Clouds and Silver Linings (Truly boring album. Two half decent mini epics sandwiched with a lot of meh. A mild improvement over it's predecessor, but not by much)
13. Systematic Chaos (I never listen to this album any more. Other than The Dark Eternal Night, I can't think of a single song I care to revisit. ITPOE is easily their worst and most forgettable epic. Part 1 is good, part 2 is too hard to stifle laughter to bother with. If it was a Spinal Tap album, it would at least have some comedy effect....but considering they were serious just brings it down)