1. Octavarium - I always liked it to some extent, but it took some time to fully grow on me, but now, yeah, it's one of the most enjoyable listens for me from DT, through and through. My favorite of their mega-epics.
2. In the Presence of Enemies - This is just one that I think is incredibly awesome and consistent, moment to moment, and all the way through. I don't mind that it's divided into 2, bookending the album, but in general, that does make me treat it as two different songs.
3. Illumination Theory - This is like the opposite of In The Presence of Enemies, I feel like that silent break in the middle could have warranted splitting it into 2, even though in terms of how it flows, it does feel like one cohesive whole, at least lyrically. Either way, I listen to it quite consistently, and really enjoy it.
4. A Change of Seasons - Now, here's one that's like a ringer for the epitome of DT's mega-epics, and after I first heard it, I worshipped it as much as any standard DT fan. But over time, I've grown a bit more lukewarm to it. The only part of it that I really absolutely love is Another World. Everything else is pretty okay, the main riff is nice. On its own merits, it is an amazing epic, but it's far from their magnum opus. They've topped themselves time and again since.
5. Six Degrees of Inner Turbulence - The problem with this one is that it's so big, and each part brings so much variety, that statistically speaking, it's very easy for one or two parts of it to not fully click with someone, and that inevitably brings the whole thing down. While About To Crash (+ Reprise), Solitary Shell, and Losing Time/Grand Finale are fantastic, and War Inside My Head is pretty cool, but for me The Test That Stumped and Goodnight Kiss (especially being back to back), really bring the whole thing down. They're not awful, but the lyrics and the execution don't stand up to the other parts. And it's unfortunate that the Overture wasn't done with real instruments. It does sound a bit inorganic.
6. The Count of Tuscany - The red-headed stepchild of the mega epics. Its best musical moments are amazing, but those are mostly the melodic, acoustic ones, i.e. the intro and the outro. The heavy parts have some cool moments, but overall, it's heavily underwhelming all around. And I'm not even one of the people who has a problem with the lyrics, just musically. Portnoy's backup vocals bring it down a great deal, but the vocal melodies in general just weren't very good or interesting. The riffs and other parts were a bit underwhelming too.