My vote goes for Renaissance. And, although I understand there are obvious omissions, why is always Mike Oldfield neglected as a top prog artist?. Even when other prog groups were releasing crap in the 80s, he continued recording quality prog albums (usually with the formula of a prog side A and a pop side B). And although he recorded one of the most popular prog albums ever (Tubular Bells) and one of the best (Ommadawn), even I usually don't give him as an example when other people ask me what prog is about. Is it that a vast number of proggers just don't consider groups that are mainly melodic and never dissonant or agressive or without a liking for certain keyboard sounds? For example, why Van Der Graff is top 10 and Renaissance and Oldfield don't?
PS: That 'keyboard sound' thing reminds me an unrelated complain I have against some modern prog: sometimes they concentrate more on recreating the 'sound' of prog than on compositions that could be at the same level as the classics. A Emersonish sounding keyboard solo doesn't meke a good prog song.