Extremely difficult to do. I've managed to whittle it down, but I can't say whether this is my definite top 10 list or not, because I could easily flip flop on several of them with others that I also really enjoy, including a couple from the MM-era (somebody wake up Bosk - I'm sure he just passed out reading that).
All that. And I also have to * my list and say that, while it is possible that a song or maybe two from d/t might make the cut, I don't typically include songs from whatever the latest album is whenever doing such a ranking. The songs are still too fresh and new. So here's my list, with a bit of explanation:
1. Six Degrees: I am conflicted about this. It does have a few parts that I'm not in love with, whereas my #2 and #3 choices are solid from start to finish. So I kind of feel like #2 and #3 are more solid. But I just boil it down to this: Sometimes, more is just more.
2. A Change of Seasons: If we are limited to studio versions, this song might actually slip down a couple of spots. But the live performance on LSFNY is perfection. The passion of that performance was and is a defining moment in my DT fandom and holds a VERY special place in my heart.
3. Octavarium: Perfection from start to finish. And I love how the song explores its theme in so many different ways and gets so "meta" in different levels.
4. Metropolis: Pull Me Under was the first song I heard and the first song that hooked me. Metropolis was the first song from them that really bent my mind and got me to think about music in a different way. In many ways, it was and is the "definitive" DT song. It really captures everything the band is about.
5. Lines in the Sand: This song really brought out a different side of DT. And I realize that a lot of that was Derek's influence and a perfect storm of events. But sometimes, that's how great songs are written.
6. Scarred: The herky-jerky transitions really put me off this song for the longest time. But once I got over it, the raw power and many layers of this song really floored me.
7. Bridges in the Sky: Such atmosphere, and so many emotions captured musically. I just love it.
8. The Mirror: Simple song, by DT standards. But it is powerful and effective. I love the version on LSFNY with the bits that became Lie re-introduced back into this song. That really took it to another level for me.
9. Trial of Tears: So many moods in this song, and all of them cool. This is one of those songs that simply paints a lot of different pictures in your mind and is more about moods than particular subjects, and it works.
10. Home: That whole intro leading up to the first vocal passage is one of my favorite DT moments of all time. The verses and chorus are fantastic. The instrumental break really takes you on a journey, and I love how, by the end of it, you are about a million miles away from the main musical themes of the song, and them it seamlessly brings you right back. If not for the porn samples, this might be top 5.
Some honorable mentions: Take Away My Pain, The Glass Prison, In the Name of God, Panic Attack, Forsaken, Constant Motion, A Nightmare To Remember, Outcry, Beneath the Surface, The Bigger Picture, Behind the Veil, Along for the Ride, S2N, At Wit's End