When I think of what constitutes a "masterpiece", there is generally one album by a particular band that stands out above the rest. For Dream Theater, that was Images and Words as far as I'm concerned. Each and every second of that album is pure brilliance without a single dull moment. Hell, even the leftovers from I&W like Don't Look Past Me and To Live Forever are amazing songs. There is just something special about that album, something about the band being young enough to still have something to prove yet old enough to have just enough maturity to their songwriting, that makes it one of those once in a career works of art. In my mind, they will never top it, no matter how hard they try.
With that said, however, Dream Theater is one of very, very few bands for whom I can say that I often listen to each and every song they have ever written and enjoy it all. As far as I am concerned, DT has no bad albums. They have never had a point in their career where I felt they lost their way musically. I have always felt comfortable simply buying a DT album blindly upon release because I trust the band and their quality of music, and I am always ready for them to take me on a brand new musical journey, where ever that may lead. That is not to say that I love all their albums equally, but when you're talking entire albums, even DT at their weakest is better then most bands at their strongest.
It seems to me that the qualification for a "Masterpiece" in this poll is something like "the last Dream Theater album you absolutely loved from top to bottom". If that is the case, then I am voting for the second option. Over all, I feel the band was just as strong in the last decade as they were in the 90's, and even with the new drummer, I don't feel the need to hear samples of their next record before the release. If it has the Dream Theater name on it, I will buy it, and I know beyond a shadow of a doubt that I will thoroughly enjoy it.