I'll vote Six Degrees, Octavarium, A Dramatic Turn of Events and Distance Over Time because that was my top four when I last ranked. However, this is one of those things that could easily be different day to day, which I definitely feel is one of the strengths of the band. I honestly don't think I have major gripes with any DT album, at least to the point where I wouldn't consider it worth listening to for pleasure's sake.
These days, and for a while now, I always worry when I really love new music immediately on first listen, especially with prog rock music, because after the honeymoon phase you might feel it isn't as good as you first thought, whereas my favorite prog music grew on me over time.
I find this line of thinking intriguing. There does feel like there's a certain level of validity to this (I have warmed quite heavily to DT's balladry over the years for instance, despite an initial distaste for them), but at the same time, I think it's a tad more nuanced. To me, I see it as four different things: either the album just doesn't grow on you no matter how hard you try, or you don't like it at first but it begins to grow on you, or it hits hard at first and begins to fade or it makes a great impression and builds on that with further listens. I wouldn't necessarily be worried about music that's a bit more immediate, particularly when it's brimming with nuance and depth even on early listens. I find that this tends to happen with genres I'm more familiar and comfortable with (after all, The Glass Prison's impact I don't think has faded with me, even with its strong first impression), while the growers that take hold after an uneasy first impression seem to usually be from styles I'm not used to.
I disagree that there was less metal on albums like ADTOE and TA, like there was on 8vm.
wut
But like... there was, though? If not ADToE, then definitely TA. Maybe it could be reasonably argued that if ADToE had the production of an album like Black Clouds or Distance Over Time, that it'd be considered a much more metallic release but even then, I'd still say less so than either of those two. With The Astonishing though, metal is pretty much entirely absent for probably well over the majority of its runtime.
Talking specifically about prog, I can totally enjoy a first listen as an experienced prog rock listener. If I don't wrap my head around all the music on the first go, and realize I'll need to dig deeper and explore more to 'get it', that is good. However, there has to be something that catches my ears on first listen to help remind me of why I need to go back and listen again. If I enjoy it the first time, but nothing particularly stood out, or I get bored on subsequent listens, that is not good. The Flower Kings are a good example. It took me years to get into them, after sampling some of their music here and there for a while, but I knew there was something about what they did that I liked, what with being into Yes, Genesis, Zappa, Crimson, ELP, etc... and liked their take on the genre. At some point, the planets aligned and I got it. Then I couldn't stop collecting their albums. I knew I'd like TFK, it's just I hadn't had that "a-ha!" moment yet. Whereas with other bands where I get it right away, I lose interest fast. Stuff like Porcupine Tree or Steven Wilson's solo albums, or Riverside, don't get me wrong, it's not the mood of their music, it's that I don't find anything that keeps me wanting to come back, aside from a song or two here and there.
Mangini-era DT albums are like that, too, especially ADTOE. I listened to that album a million times when it came out, and I still like it, production aside, but I don't hold it in high regard as I did in the months after its release. I really got into DoT when it first came out, too, but shortly after, I realized I only liked, not loved, two or three of the songs...
As someone who knows metal well, but barely listens to metal anymore, The Astonishing is definitely metal. Maybe not as much as the releases surrounding it, but it's still metal, and more metal than Octavarium the album. 8vm has only 3 songs that are metal, (TROAE, These Walls, Panic Attack) While it has its moments, Sacrificed Sons is closer to heavy rock than actual metal IMO, same with Never Enough. The title track is not metal AT ALL, unlike all their other epics.
Sure, there's more piano, more balladry, etc... than the average DT album on The Astonishing, and sure, there are lots of other songs and non-metal moments on the album, no doubt, and sure, it's more whimsical and flamboyant than the usual DT fare, but pound for pound, the album is loaded with distorted guitars, low tuning chugging riffage, hard hitting drums, you know, the band doing metal things, even if it's just power chords. It's more that the band falls back on that sound more than anything else throughout the album and kind of makes the listen a chore, among other reasons, when they aren't doing the other things that are on the album, like leading so many tracks with slow, lone, sad piano.