What about Beefheart? Or Zappa? They're not considered "prog rock" artists, but they're arguably more progressive than any of the aforementioned bands.
This is really splitting hairs on how to define prog, I think. Arguing someone is "more progressive" than the others means that, like, Diamandas Galas is more progressive than YES because they're still using traditional time signatures and a guitar/bass/drums combo, even if they're dropping past the verse/chorus/verse form.
All the early GENESIS records made me a fan ("Nursery Cryme" through "A Trick of the Tail"), then SOFT MACHINE clicked, especially "Third" and "Fourth," after years in the jazz desert. Canterbury stuff is a trip, but probably not the most easily accessible.
Soft Machine Fourth is one of my all time favorite records but I always assumed it was outside of the prog canon because of how jazzy it is.
I think it's probably where the first big wave of artists pushed up against their influences and started to go more into exploring one influence over the other? That feels incoherent, but, like, Soft Machine drops so much on that record and just goes for it. But all of the Canterbury stuff I've heard (Soft Machine, Gong) seem much more willing to incorporate jazz into the mix and when I think prog I guess RUSH would never cop to having Don Cherry and Ornette Coleman as influences. I guess more thorough definitions are just needed?
I feel like I don't have a TON of knowledge on prog (certainly not as much as the poster talking about MAGMA), but to me it boils down to, like:
THIS IS PROG:
https://www.youtube.com/watch?v=Ne317y_eOYsTHIS IS KRAUTROCK:
https://www.youtube.com/watch?v=390Nz5Gm8Hw&index=12&list=RD8QLL2j8ZtxEI don't know. They're so different to my ears.
Yes I know this is the most message board-y post of all time.