Images and Words, Six Degrees of Inner Turbulence, Octavarium and Dream Theater actually form my entire top four, so I didn't have to spend much time thinking about this. Their most colourful and creatively fertile albums. BCSL and FII aren't amazing, but FII's got a lot of fantastic songs, and while BCSL is one of the more mediocre DT albums, that's like saying, "It's one of the duller sexual positions." You're still going to have a pretty good time.
My favourites of the odds are probably A Dramatic Turn of Events and Systematic Chaos - both phenomenal, but neither quite punch with the best of the evens. Awake, Scenes and Train of Thought are good, solid Dream Theater albums, and When Dream and Day Unite wants to be a good, solid Dream Theater album, too. It's a robust group, but nothing's good enough to make me think twice about picking the evens.