Music is so subjective, and there are so many different and uncomparable genres, that it's kinda hard to have a single favorite song to be considered "greatest of all time".
I can tell, echoing some of the posts before me, that I can call Scenes from a Memory my "favorite album ever", but then again it's an honorary title because:
- I heard it in my formative years of my tastes in music
- I had the immense surpise of seeing it in the store without knowing anything about before, huge surprise
- It is actually a mindblowingly and godly awesome album
But I claim it's my "favorite album ever" because of those reasons, the memories associated with it, it's not that I listen to it regularly after 20 years, this year I heard it exactly once, and it was when it was played live at my concert.
I can also agree that the ending of Octavarium, especially live, gives goosebumps, and all in all my personal favorite DT song is Metropolis (podium is completed by A Change of Seasons and Voices). But I can't bring myself to take it to the next level and label any song as the "greatest of all time".