BIG FIVE

Imo, just because Pink Floyd were psychedelic / space rock band, they must be in the Big FIve. I mean, Pink Floyd in Big 5, with their vast catalogue, are covering all that huge genre with all of its sub-genres -  from early English psychedelia to the progressive psychedelia.

I can't really talk about Tull all that much as I only own Stand Up , Aqualung and Thick As A Brick. I am aware that Passion Play is regarded very highly but then there are another 4 essential prog masterpieces (in your estimation) that are not discussed as much on the forum and I can't say I'm aware of their status in this regard.

ELP made some poor decisions after 1974 there is no doubt but at least they didn't just repeat themselves and they do have a couple of later albums in their catalog that are highly regarded ( ELPowell and Black Moon). I don't think this matters too much though as the era of innovation in prog was 1970-1974 after which many prog bands either just repeated themselves or took a more commercial stance with rare exceptions such as King Crimson (but not Tull I suspect)

Imo, ELP's greatest are their first three album and Works Vol 2. I mentioned Brain Salad Surgery because that album is, if i am not wrong, widely concerned as an essential prog masterpiece.

However, ELP's catalogue is not so large as Jethro Tull's, and ELP don't have so much essential prog masterpieces in their catalogue that to be enough to take Jethro Tull's place in Big 5. I mean, if some band deserved their place in this Big Five group of the great bands, that supposed band have to have a larger catalogue than e.g. ELP.

Also, that supposed band ought to be way more popular than Camel, VDGG and PFM - the great bands you mentioned; Jethro Tull was / is more popular than these three bands altogether, imo.

It is obvious that many great ones are as deserving or more than those five, but those five are what they are.

Another thing is trying to make a list of newer Prog bands which could be considered as the modern big ones, but when there's no element of innovation, quality alone is often not enough to bring a band on such a pedestal. Personally I am not too in depth with much modern Prog but I certainly see bands such as Marillion, IQ, DT or the Flower Kings as deserving a status as the "architects of the revival of Prog". 

King Crimson were never really a big band. Influential, yes, but no more than a large cult band. Apart from the debut and In the Wake (which most of us bought on the back of the debut and mostly absolutely hated), their albums scraped round the lower reaches of the UK charts.

The others were huge, as were Tull. I think Rush also have to be included now as from the late 70s on, they were comparable to anyone in the genre.

Yes. According to progression magazine, is "The Big Six" of progressive music, not the big 5,  and Tull is in the list.