I've never been a huge fan of the debut. Between Dominici's voice and some less than memorable song structures, it's not an album I pull out very often at all.
Images and Words is the album I voted for, and the one I always come back to time and time again, loving every single song.
Awake is certainly a great album with one of their best songs, "Space Dye Vest," but there are a few songs I don't love. This is definitely DT's best album for sound quality.
FII is a two-face album for me. I absolutely love some of the songs, whereas a few are among the band's worst.
I always have to digest SfaM as an entire album, and as such, it works wonderfully. But I don't adore each and every single piece on its' own, and that reason alone ranks it beneath IaW in my book.
6DoIT, much like Awake, is a great album, punctuated by a few spots of mediocrity. Wheras those spots are 5-minute songs on the latter, on 6DoIT they average much longer.
ToT is the last great DT album in my book. There are a few previews of their coming decline, but for the most part, it is a superb, and HEAVY album.
8varium is where I began to dislike DT. Between boring long noodling, insincere sounding soft parts, and awkward vocal melodies, I didn't like much about this album from the get-go. And I still don't.
Systematic Chaos is like a 3rd rate ripoff of ToT. While at first I enjoyed the heavier sound as compared to 8varium, I quickly grew sick of the blatant Metallica cloning, Portnoy's horrid vocals, and the band's worst-ever lyrics.
BCaSL showed a bit of improvement over the last few efforts, with a few tracks that actually please me. It's still quite a bit off their pre-8varium levels, but at least it gives me hope that DT might put out a great album again someday.
My DT album ranking:
Images and Words
Scenes from a Memory
Train of Thought
Six Degrees of Inner Turbulence
Falling into Infinity
When Dream and Day Unite
Black Clouds & Silver Linings