From what I've managed to discern on this forum, DT forum, some reviews, prog blogs, prog lists and Mike Portnoy tweets, this is some general shaped opinion:*
*Mind you, I haven't listened to the whole discography, some of these sayings are biased and other ones are made up
When Dream and Day Unite - Don't even bother. I can't remember if I stumbled upon on even any band member saying something postive about this one.
Images and Words - TNOTB of Dream Theater, I guess? Half iconic, half filler.
Awake - It is amazing how overlooked this album is. I mean, this one rarely gets mention. I don't know is it a good thing or a bad thing.
Falling Into Infinity - It's not Dream Theater's fault that you will not like this album. It's the music industry's fault.
Metropolis pt 2. Scenes from a Memory - Masterpiece, apparently. I mean, it was voted best concept album recently, topping 2112.
Six Degrees of Inner Turbulence - There's something for everybody here. And I do mean: something for everybody.
Train of Thought - If you're born like like... 1985 onwards, you will like this one. Otherwise, you won't
Octavarium - Title track is what's this one all about.
Systematic Chaos and Black Clouds... - Not good as people expected them to be.
Dramatic Turn... and Dream Theater - Not bad as people expected them to be.
The Astonishing - If you ever become, like, hard-core DT fan, this one will end up in your top 2.
Distance over Time - Lukewarm. Nobody bashed or praised. Switzerland of DT fandom.
I actually got through a fair chunk of the albums, have a few more to do but I quite enjoy the band and probably have them playing far more frequently than any other band.
Images And Words - Phenomenal album, I only really go for 3 songs right now but wow what a release.
Awake - Has some pretty darn strong tracks, lately I've only been listening to
Lifting Shadows Off A Dream but I do recall the second half of that album being super strong.
Falling Into Infinity - Not a ton that I listen to from here, once again another consistent and stronger album.
Hollow Years is what I listen to from here.
Metropolis Part 2: Scenes from a Memory - Absolutely amazing, only 1 mistep imo, a brilliant display of their craft,
Home, The Spirit Carries On and Finally Free all get played regularily.
Six Degrees - A pretty good album which I haven't listened to anything off of since, but I did like it, albiet a very long album.
Train Of Thought - A good album, another that I don't listen to much from, the band took a more heavy approach here which while it works wasn't my favourite.
Octavarium - Another rather good album which I once again haven't looked into much since. I liked a fair bit on it including the title track.
Systematic Chaos - See above, I do like some of their stuff from here every so often,
Ministry Of Lost Souls is a great track.
Black Clouds And Silver Linings - A brilliant album, I love it.
Wither I listen to almost every day,
Best of Times and
A Nightmare To Remember are also some which get a fair bit of listens.
Haven't listened to the rest. Oh and
A CHANGE OF SEASONS has been played a lot lately, much to my familys disapointment since they aren't a fan of longer songs or rock/metal in some cases.