Here are some honorable mentions, which I haven't seen posted yet.
Tangerine Circus / The Conspiracy (20:18) A gem by the band where DarkLordLalinc is the writer and keyboardist. Production is a little thin, but this is absolutely fantastic. Gives me vibes of The Odyssey
Symphony X / Divine Wings of Tragedy (20:43) Has this really not been mentioned yet??
Maestrick / Lake of of Emotions (21:03) I'd be surprised if many other than RJ know this one. Maestrick is a pretty eclectic band
Jethro Tull / Thick as a Brick Part 1 (22:26) I think this is Top 5 from the >20min songs from the 70s. Personally, I'd put it 2nd to Supper's Ready
Daydream XI /
The Grand Disguise (23:41) Here you go Tim, a song you've heard of. Anyone who mentioned ACOS as their fave, I highly encourage you to have a go at this one. So much so, I've linked it. Seriously, do yourself a favour.
Moon Safari / Lover's End Pt. III - Skelleftea Serenade (24:21) Peak Moon Safari afaic
Dream Theater / In The Presence of Enemies (25:39)
Spock's Beard / Falling for Forever (27:07) I mean c'mon... all three eras in one song!
Southern Empire / The Bridge that Binds (28:22) This is filled with catchy hooks, and neo-prog greatness
Transatlantic / Kaleidoscope (31:54) the biggest TA song, and in my Top 3 by them
Shadow Gallery / First Light (34:19) Sure, it's really only a 30 minute (ish) song, but it's 30 minutes of awesomeness. RIP Mike Baker
Just in case anyone's interested, here's a
Spotify Playlist of these (sans Spock's and TA, cuz we all know how much Neal hates Spotify).