2112 by Rush. I've only listened to the title track, however, none of the other songs on that album. It might be 20 minutes long but it never gets boring, there's a lot of variation between and within the different movements.
I just as easily could have said Their Satanic Majesties Request by The Rolling Stones. It has two of my favorite songs: Citadel and 2000 Light Years from Home. The sound of those songs is in my opinion really cool and unique.