Okay, I'll play and do my personal rankings as well. Preliminarily, I don't dislike any DT album, so even the albums that are ranked near the bottom are albums I would consider good or even great. I won't separately rank the live offerings, since they are just so varied in terms of format and what they offer that it is difficult to compare some of them meaningfully.
1. Six Degrees - The title song is among my favorite songs of all time. And looking through the rest of the tracks, the only low point for me personally is Blind Faith. Otherwise, I love every song.
2. SFAM - The sum is definitely greater than the parts. This album was both the rebirth of DT and of my fandom of the band.
3. Images & Words - The album that really got it started for the band and for a lot of fans. There are some absolute classics on this album. A couple of songs have never really clicked for me, or this one might be ranked higher.
4. Octavarium - Although it has a few low points, the album is consistently good from start to finish and concludes with one of the top 3 songs in their catalog by most fans' standards. While not a traditional "concept album," the album theme in all of its various incarnations takes the album to another level. If I&W wasn't such a classic album, this could easily be #3.
5. Black Clouds - The only weak point on this album is TBOT (which is a very good song, but just doesn't speak to me on a personal level) and perhaps the lyrics (which aren't bad by any stretch, but aren't as deep and meaty as on some of their previous efforts since they are largely narrative).
6. Awake - Chock full of even more classics. But some weak points as well, in terms of a few weaker songs, and in terms of sonics (such as Kevin Moore's odd choices in keyboard patches, and the overall sonic quality of the album, which just sounds a bit jarring the first few times you listen).
7. Falling Into Infinity - This album took me a long time to get into, but has some really solid tracks. Most of the decisions between demos and final were good, but a few left the album lacking (the final version of TAMP; not including Raise the Knife).
8. Train of Thought - For DT, it felt a bit one-dimensional, and can leave the listener feeling exhausted by the time it is over. Also a few moments that detract from greatness, such as the "Blackened" part of TDS and the instrumental section in ES. But still a solid effort that is a great part of the DT legacy.
9. Systematic Chaos - A good album, but very inconsistent. ITPOE1, Forsaken, and Costant Motion are hands-down some of the best things the band has ever written. Other tracks really drag the album down for me, such as Prophets of War (which I really do not like at all), TMOLS (which just feels too "by the numbers" and has an instrumental section that doesn't fit and detracts from the rest of the song, and Repentence (which is a really good song, but is far too one-dimensional for how long it is. ...or is far too long for how one-dimensional it is. Either way). A solid effort, but I only really regularly want to listen to about half the album.
10. WDADU - Some really, really good ideas. Some things worked. Some didn't. I like Charlie hearing him do his own material. A lot of DT's material simply isn't written to his strengths and doesn't sound right. Add the production issues and a few issues that really showed that the band was experimenting and just going for it, and the album loses some points. Granted, some of that experimental "forget about the rules" vibe also adds points. But this is easily for me the lowest point in the catalog, even though I would still call it a "good" album.