This site is ambitious, fascinating, and not always successful artistically, much like prog rock itself. Like Vinyl Surrender, another site that I like, the rankings are based on site member votes, so the list is not static. Unlike Vinyl Surrender, this is a deep, deep dive into one musical genre and it's many sub-genres. In addition to seeing the top prog rock albums, you can subset by subgenre to just see the top albums from the Canterbury Scene, or from Krautrock, or from RIO.
As of today (Sept 11, 2011), the top 10 prog rock albums on the site are:
10) Godbluff - Van Der Graaf Generator
9) King Crimson - Red
8) Pink Floyd - Animals
7) Pink Floyd - Dark Side of the Moon
6) King Crimson - In The Court of the Crimson King
5) Genesis - Foxtrot
4) Pink Floyd - Wish You Were Here
3) Genesis - Selling England By The Pound
2) Jethro Tull - Thick as a Brick
1) Yes - Close To the Edge
I have to say that I'm disappointed that even when I subset on Rock In Opposition (RIO), Henry Cow's "In Praise of Learning" only comes in at #115. Keep in mind that the list changes as the votes come in, so it may be a little higher or lower than #115 when you read this. Their later album "Western Culture" fared better, but I've always preferred the three sock albums, and I've always felt that Henry Cow was the archetypal RIO band more so than any other.
Prog rock has a lot in common with Metal in terms of being a critically maligned and/or ignored genre. They are also genres with relatively few female fans and overwhelmingly white audiences. Maybe rock critics, who are also disproportionately male, are trying to hide from their music geekdom by rejecting these genres.
There was a good bit of prog rock on my best of the 70s list, but I also left out some major prog rock bands entirely. I'm not entirely sure what makes the difference between complex and challenging RIO that's enjoyable and complex and challenging commercial prog rock that's annoying, but this site has plenty of both to explore.