The best prog album of all time?

none of the usual suspects. there are so many better ones that unfortunately are too obscure. I would vote for "Fairy Tales" by Mother Gong

We have to understand what it means best prog album of all time. 

It could mean 

1) the most progressive opera, or 

2) the most beautiful progressive opera. 

That is to say, there is a FORMAL aspect of respect for those canons that make "progressive" some rock music, and there is an AESTHETIC aspect that relates to the beauty of music, and that is transverse, not only about Progressive, but all kinds of music. 

We must talk about what is art.

What did Lukacs say about Emile Zola, "naturalist" writer?

Naturalism is a precise type of literature, which had as a program that of transforming the novel into a scientific experiment, where the characters moved according to the environmental Conditionings and Genetic, that is, according to Darwin's theory. Scientific positivism was the soul of naturalism, and therefore Zola was striving to narrate a story in a completely objective way, as a scientist, without putting any romance, without representing any hero but only people who corresponded to the average Statistics. 

Well, what did Lukacs say about Zola? He said that his naturalist art, so conceived, limited the beauty of many of his works (the desire to respect certain formal canons limits the aesthetic beauty) but fortunately Zola often, unintentionally, broke the canons of naturalism, and dashed his Characters not in a goal but romantic, getting involved, and precisely for these violations of the canons of naturalism he has achieved great results as a writer or he has written very beautiful novels. 

Behold, I believe that this applies to every art, including prog: the most progressive work, the one that respects every canons of progressive rock (changes of rhythm, long songs in suite style, instrumental solos etc.) will hardly be the most aesthetically Beautiful. It will be the most "proggy". But the most ingenious, original, inspired, in all likelihood, will be a work that has not been written according to the standards of progressive. 

For example, every albums that you have quoted is very proggy. Classic prog. Are they the best prog albums of all times? I would answer No (but Pawn Hearts, in my opinion, could be candidate, together with In The Court of...). Instead, I believe they represent in a perfect way the canons of prog.

In particular, "Close to the Edge" is a very representative work of progressive, it respects all the canons and the patterns of prog, it's perfectly proggy, but in my opinion it is aesthetically much less beautiful than a work like "Rock Bottom", which none of us would take as a perfect example of progressive work. 

Bruford's drumming on the album is under-appreciated in my opinion, it is some of his very best work!  

I saw them on the Yessongs tour, Sept 22, 1972.   I was expecting to hear the AM radio hit "America" and hadn't heard a single song off of CTTE, so imagine my surprise!!  They played like the devil was chasing them.  

There are many worthy albums to mention (Foxtrot, SEBTP, etc.) but they all have some minor weak point or slow song....CTTE had none.  

In the court of... is also a contender. Five great songs, unfortunately moonchild's 9min jammin section gets sometimes a little boring. Same problem with providence in Red. Maybe Larks then?

Camel - Mirage?

As you said, CTTE checks all the categories and could be the one :)