5 Six Degrees of Inner Turbulence. My personal favorite dream theater album, there's just so much greatness on it, Misunderstood, Disappear, the Great Debate, of course the 7 part 42 minute suite that is Six Degrees of Inner Turbulence, and my personal favorite the Glass Prison.
Images and Words is by far my favorite. It's not the most accessible album in the world and went completely over my head the first time I heard it (probably didn't help that I hadn't heard much progressive music at that point), but once you give it a chance to sink in, holy crap is it good. Everything they did in the 90s (except for a few songs from Falling into Infinity) is really good, so you can't go wrong with Awake, A Change of Seasons, or Scenes from a Memory either (I like FII but wouldn't start with it).
But Images and Words is the best. As incredible as Images and Words and Metropolis Pt. 2 are, I personally find Awake to be Dream Theater at its finest when it comes to their musical prowess.
It's just so. Flawless to me, so Awake is my top recommendation. Some may be put off by its darker themes, but I really like everything about the album.
Images and Words is another must-listen, and James LaBrie absolutely nails it in that album with his incredible vocals. Don't forget their A Change of Seasons EP, either.
The title track is a masterpiece, of course, but they have some live cover songs that are awesome as well. While I enjoy DT's earlier works more, Six Degrees is another great album, as is Train of Thought if you're okay with them going for a more 'all-out' metal approach with dark themes. Images and Words. Metropolis Pt. 2 - Scenes From a Memory.
A Change of Seasons Then take some time to read about (it's a selection of 5 DT songs over 5 of their albums, inspired by Portnoy's prior addiction to alcohol). Then listen to Octavarium; the album itself isn't anything special IMO, but the song Octavarium itself is epic. Listen to it once, then look up the meaning behind the song and listen to it again;actually i think does a good job. After that, check out their live albums, ESPECIALLY Live At Budokan, and check out the DVD if you can. Also, in 2006 they recorded a 20th anniversary concert, called Score, and it's pretty great IMO.
They played some great songs, they played the entire 8-piece Six Degrees Of Inner Turbulence saga, which I also personally like. They also play Octavarium (the song) live, which I really enjoyed. If you get to that point, you're probably a big fan and you'll just wanna get your hands on everything of theirs.