My favorite progressive rock albums of all time are Pink Floyd's Wish You Were Here, Red by King Crimson and Blackwater Park and Ghost Reveries by Opeth.
And although it just hints at prog, I have to throw in Metallica's ...And Justice for All. Not progressive in the traditional sense, it's Metallica's most progressive release, and I love it. Even though it lacks bass guitar :-)
As others have stated here, it's hard - if not impossible - to pick THE best prog album of all time. I am wondering, though, based on the suggestions so far, whether we are talking about the best prog album or the best concept album? In some ways these are the same thing imo. Prog bands mostly defined the idea of the concept album - at least until Green Day came along with American Idiot (which, by the way, was a pretty decent album for what it was).
The Wall, 2112, The Lamb Lies Down on Broadway, etc. are all concept albums by prog bands.
So, if that's the case, I would look at concept albums and nominate one I think has a great story.... Operation:Mindcrime from Queensryche.
In terms of life changing impact and emotional connection, I have to pick ' Selling England By The Pound'. This one still brings tears to my eyes every time I hear it. 'The Lamb Lies Down On Broadway' is less of an emotional choice but more filled with wonder and experimentation and, of course, Foxtrot. 'The Court Of The Crimson King' has to be the most influential in terms of its impact on the direction of music. Others in no particular order: Pink Floyd-Dark Side of the Moon, The Wall, Animals, Wish You Were Here, Yes-Close to the Edge, Relayer, Going For The One, Camel-Rain Dances, Moonmadness, ELP-Trilogy, Steve Hackett-Voyage Of The Acolyte, Brian Eno-Another Green World, Mike Oldfield-Ommadawn, Supertramp-Crisis, What Crisis?, Nectar-Remember The Future and Recycled, Fripp and Sylvian- The First Day, Strawbs-Hero And Heroine and Ghosts, U.K., Tangerine Dream-Phaedra....... the list goes on....
Tough one! But I am in aggreement with some others that "The Lamb Lies Down on Broadway" by Genesis Is kind of the pendultimate of prog. Nothing seemed the same after it.
...Also I enjoyed the band Gryphon's "Red Queen to Gryphon" is about as good as it gets.
This got me to thinking. For pure musicianship and creativity,I would like to add Gentle Giant's " Three Friends" into the mix. The interweaving patterns and melodic weavings make this a breathtaking journey.
I would have to say the "best" album as far as music, sales, and impact is Darkside of the Moon by Pink Floyd. My favorite prog rock album is unknown, I am very fond of Wish You Were Here, The Lamb Lies Down on Broadway, and an ocean of albums. But Pink Floyd put prog rock on the map in a huge way with Darkside, and they kept prog rock on the map the last 40 some years.....