Let me see if I can get this right. My top five are mostly set in stone (2 and three occasionally swap), as are my bottom three, but everything else in the middle is very fluid.
1) Scenes From A Memory- Still probably my all-time favorite album from any band, and the record that turned me into a massive Dream Theater fan. Pretty much flawless from front to back.
2) Images & Words- I agonized over this vs my next entry on the list, but in the end I&W wins out by a hair. Metropolis is the quintessential Dream Theater song, and Wait for Sleep/Learning to Live is a phenomenal album close. I feel like Take the Time and Under A Glass Moon get overlooked sometimes for sharing an album with those tracks, but they're pretty great in their own right.
3) Awake- I actually prefer the darker, heavier vibe of Awake to Images and Words, but the heights of Awake (Voices, Scarred) are just short of the best I&W material, and Caught in a Web is much more of a filler track than anything else on these two albums. Lifting Shadows Off A Dream is a hidden gem.
4) Six Degrees of Inner Turbulence- The first Dream Theater album I ever purchased, after someone on the Metallica forums wouldn't stop raving about this song called The Glass Prison. I popped this into my CD player, and right about when the double bass for TGP kicked in was pretty much a musical epiphany for me. I actually took a while to warm up to the rest of the album because I wanted everything else to be as heavy as The Glass Prison, but over time as my musical tastes expanded/matured beyond just "heavy", I came to love the rest of 6DOIT almost as much as the first track.
5) A Change of Seasons- With only one track of original material on here, I couldn't bring myself to rank this in the top four. That said, ACOS still stands up for me as the greatest song Dream Theater's ever recorded. The cover medleys are pretty cool as well, I've always been enamored with the transition they pull from Carry On Wayward Son to Bohemian Rhapsody.
6) Dream Theater- My favorite of the Mangini era albums. It doesn't quite reach the highs of ADTOE, but start to finish it's a stronger, more consistent album.
7) Octavarium- Gets here almost on the strength of the title track alone, though I do like the rest of the album as well, especially the first four tracks.
8) A Dramatic Turn of Events- While not an all-time classic Dream Theater album, a solid first offering from a band with a new drummer and writing chemistry following the departure of Mike Portnoy. Bridges in the Sky is a blast, and Breaking All Illusions is a great throwback to the Images and Words era DT sound and better than any song they had put out since Octavarium
9) The Astonishing- Still digesting this one to be honest, but I dig the change up in writing style and I love Labrie’s performance on this one. A little bit bloated though, and the actual story is laughably cheesy.
10) Falling Into Infinity- It pains me to rank this so low because of how strong I find Trial of Tears (top 10) and Lines in the Sand (top 15), but aside from these two and Peruvian Skies I very rarely find myself listening to this album at all (Hollow Years doesn’t count as I always go with the epic Budokan version).
11) Black Clouds and Silver Linings- I find this to be markedly better than its predecessor and so was a step in the right direction for the Portnoy version of the band, but still pretty unremarkable by Dream Theater standards.
12) Train of Thought- The first new release after I became a fan, and I was primarily into metal at the time it immediately endeared itself to me. Age has not been kind to it for me though, as I mostly only listen to it now when I’m working out and want some riffage and shredding. Way too one dimensional of an album.
13) Systematic Chaos- I probably spin this a little more than some of albums above this, but it's the Dream Theater equivalent of junk food for me, there's just not much of substance there.
14)When Dream And Day Unite- Probably the easiest spot on the list for me to fill in. Every now and then I'll listen to a live cut of The Killing Hand from a pre-food poisoning Labrie show, and I like to spin the Score version of Afterlife on occasion, but I couldn't tell you the last time I had the urge to listen to any of the WDADU studio tracks at all. This is basically a once every few years spin for me to see if my opinion has changed (spoiler: it hasn’t).