Dream Theater have exactly 4 discs worth checking out, IMO:
Images & Words
Awake
A Change of Seasons
Scenes From A Memory
Images & Words is pretty much what you'd expect from an early 90's progressive metal album (lots of lingering 80's stylings), just done very well. If you don't find the 80's feel incredibly cheesy, it's all rather tastefully done.
Awake feels more 90's. It's all around much heavier than I&W (though not as much as ToT), and the lyrics are less abstract and dreamy and more dark and spiteful. It has the only 'angry' lyrics I've heard from Dream Theater that felt genuine to me; starting with 6DOIT, they began trying to sound pissed off a lot more often all of the sudden and it feels really forced IMO. Awake gets the feel right.
A Change of Seasons is a 20 minute studio track and a few live covers. The studio track itself is a favorite among most DT fans (I personally think it drags in sections) and the covers are really good. Best live vocals I've heard from the band.
Scenes From A Memory is a concept album and really proggy. It's pretty heavy in spots, and very gentle in others. A brilliant album musically and, aside from one or maybe two tracks, has DT's best studio vocal performance IMO.
Awake and Scenes are my favorites.
Oh yeah, the whole album is great. The more I think about it, the more I think I can easily say it is by far their best album.
It does have "The Silent Man" though, which I don't mind but it's pretty much the definition of a ballad. All DT albums have one, though. Even Train of Thought, IIRC.