The "20+ minute" limitation means I can't use Close to the Edge or Cygnus X-1 Book II: Hemispheres.
Agreed. I think if the thread was specifically "Side-length" epics, it would not only allow songs like CTTE or Hemispheres a place, but also narrow it down to songs that probably don't reach over 26-27 minutes long, due to the limitations of vinyl sides, so we probably won't get songs like Neal Morse's "World Without End" or DT's "Six Degrees Of Inner Turbulence" on here.
As for strictly 20+ minute epics, "Supper's Ready" is always a favorite, but as far as modern prog goes, the bookending epics on Transatlantic's Bridge Across Forever are just benchmarks for me when it comes to symphonic prog. I also really enjoy Haken's "Visions" and of course DT's "Octavarium". Also got to shout out Pink Floyd's "Shine On You Crazy Diamond", with a full version clocking in at 25:37.
Probably Stardust We Are by The Flower Kings
Also a great choice, and probably my favorite TFK epic, unless we can also count "I Am The Sun", which, when blended back together into one piece, reaches 25:23.
I know I named like a dozen in my reply, it's just really hard to narrow it down to one.
-Marc.