Dream Theater should have way more excellent albums out. Sadly, they really only have one.
Scenes From A Memory is (in my opinion) their best. It's a theme-and-variations piece, and too many people don't notice this. It's technical, but not overly so. And it's technical in a good way (The Dance of Eternity). In some parts (the opening of Home), it's surprisingly heavy. The album's story (which is one of the best stories ever to grace a concept album) is incredibly cohesive, despite having all 5 band members take on the task of constructing the songs, and writing the lyrics.
The rest of their discography is...well, let me explain.
I love Images and Words. I do. But, when all is said and done, there's an awful lot of wankery in it (not technicality). And it sounds incredibly dated.
When Dream and Day Unite has some good songs (The Killing Hand, Ytse Jam), but I don't care for Charles' voice (though I don't much care for LaBrie either, but I don't despise him like so many do, at the same time), and the 80's sound is a bit to prevailent here (it was released in the 80's, so it's understandable, but I still don't enjoy it very much).
Awake has A Mind Beside Itself (Voices, Erotmania, and The Silent Man, for the uninitiated), but that's about it. The rest of the album has always sounded like filler to me, save for Space-Dye Vest. I never could get into any of the other songs on the album besides the ones I've mentioned.
A Change of Seasons (the song) rules. It's my favorite Dream Theater song, and one of my favorite songs, period. But there's only that on the EP. I didn't care for the covers/medleys. I can't rank it as high as an LP (even though it would place as high or higher than Scenes From A Memory), as it's just one song. Even though it's 23 minutes and awesome.
Falling Into Infinity-Dream Theater trying to go commercial. I love Trial of Tears, and Peruvian Skies is pretty good, but the band sounds really out of place on the rest songs. And the album is 79 minutes long. Ugh. Too much crap.
Six Degrees of Inner Turbulence was very disappointing. On every level. There wasn't anything I really enjoyed about this album, and what's depressing is that the concept for the second disc was great, but it was done very badly, and what ended up on the disc was a 40+ minute train wreck.
Train of Thought is the second Dream Theater attempt to tackling a commercial sound, while still trying to maintain elements of their original sound. It's better than Falling Into Infinity, and it's better than Six Degrees of Inner Turbulence, but it's still not very good. As I Am, This Dying Soul, and Endless Sacrifice are all pretty average songs (and LaBrie shouldn't try to imitate Hetfield; it doesn't work). It's a heavier (though not ultra-heavy) album, and the songs are slower, but they sort of plod along, and don't really go anywhere-they're just kind of there.
My two cents on the Dream Theater discography.