If "A Change Of Seasons" is DT's "Supper's Ready", then "Six Degrees Of Inner Turbulence" is their "2112"... and I'm not saying those things based on structure alone, but based on epicness. ACOS is great as it's a very tight, cohesive epic, but SDOIT is very great in that it introduces so many great themes in the "Overture", then expands on them in the rest of the song, with reprises and recurring themes... Definitely a top 5 DT song for me!
-Marc.