I can't help but agree that Magma should be high. Possibly Top 10.
If they were top ten they'd end up above greats like Camel, Gentle Giant and Van Der Graaf Generator. I don't think they great enough for that. Top 20, sure. Top 10? No.
Marillion was the most important prog band from a time where prog wasn't very important at all anymore...
Prog was unpopular, not unimportant. They were instrumental in ushering in the new wave of Prog of the last couple of decades. Given Prog unpopularity, it makes Marillion's successes even more impressive. When bands like ELP, Yes and Genesis were releasing pop-rock Marillion were releasing platinum selling Prog albums.
Yes, KC, Genesis, ELP, Jethro Tull and Pink Floyd were the ones who established the genre in terms of sound and popularity, and i'd say those should all be above Marillion...
All of those bar Jethro Tull are
above Marillion. Give me a convincing argument and I'll move Jethro Tull up a spot to be above Marillion as well.