I'm an outsider in that my favorite DT albums are the ones like Awake, ToT, 6Degrees, and 8VM. I know that's not the popular opinion.
But even i can tell that the holy Trinity of Dream Theater album are or should be: I&W, SFAM, and ADToE.
ADToE is essentialy I&W part 2, and SFaM is essentially just an extended piece of something you'd hear off one of those albums. They all contain the classic DT style of music, the same style that brought DT to popularity back when I&W was released. Therefore that is the Holy Trinity of DT albums.