I'm found out about DT with Panic Attack/Dance of Eternity
My big brother used to tell me how awesome the Dance of Eternity is and he showed me live videos of it. I purchased the song online(the Live Scenes from New York version, I couldn't find the other one because I didn't know it was "Scene 7 pt.1: The Dance of Eternity"), even then, I didn't know anything about Dream Theater.
I later played Panic Attack in Rock Band. After listening to Panic Attack a countless amount of times in Youtube, I bought the album "Octavarium". The first 7 songs were cool, but when I heard the entire title song for the first time, I was amazed, it's the first song to ever give me goosebumps and it opened my mind to a whole new world of music
Now, Octavarium is one of my least favorite DT albums and I have all of the DT discography. I pity all my Green Day-listening friends, they don't know what it's like to have goosebumps and tears when listening to music.