Well what I was trying to get at is that Oldfield's discography isn't nearly as consistently good or popular as some other groups, like Can and Henry Cow.
I'm not sure it's wise to advance quality as an argument for an artists greatness. For a start I strongly disagree, as I think The Songs of Distant Earth is his best work. I don't even think that it's a good idea to base an artists placement on the list on how high a proportion of their discography is Prog, as bands like Genesis are clearly extremely important artists in the genre.
As far as importance King Crimson is #1
As far as technique and skill I think Yes(or rush)
As far as Popularity I think Pink Floyd
How would you define importance? Technique isn't part of the criteria and if it was Yes wouldn't win. Pink Floyd may be the most popular generally, but for the purposes of this list I try keep things within the realms of Prog. Otherwise bands like Queen and Supertramp would be considered greater than lesser known bands who had greater influence, impact etc on the sub-genre.
The top 5 is pretty much interchangeable anyway.