I'll only rank the ones that I'm most familiar with and would be able to make the best judgements on.
1. Images And Words From Take The Time and Learning To Live, to Metropolis Pt. 1: The Miracle And The Sleeper, there's no question about it's place as #1
2. Black Clouds & Silver Linings I adored every song (yes, even A Rite Of Passage) on this album on my first listen, except for Wither, which was a grower
3. Octavarium It encompasses the song that got me into DT (Panic Attack), as well as the obvious masterpiece
4. Metropolis Pt. 2: Scenes From A Memory Yes, this is a MAGNIFICENT album, but to tell the truth, I'm not a fan of the "narrative" tracks that string the story together, which cuts down the enjoyment level to half of the songs (all of which are obviously amazing). Seeing One Last Time/The Dance Of Eternity and Beyond This Life live was amazing.
5. Awake My first DT album and also excellent
6. Falling Into Infinity I actually really like this album. At one time, Lines In The Sand was my favorite DT song, and Trial Of Tears is another great 10:00+ song.
I'm sure this will change in time though, since I'm a rather new DT fan.