I am amazed by the amount of people who say Waters is the best songwriter... how odd, though he is something to have in mind... I would argue that waters on his own would not have been able to write that well and he needed external inspiration (don't you think Animals, if in a way creation of Waters, is an allegory to Orwell's "Animal Farm" and therefore to the Russian Revolution, though of course that same message could be applied to many other real-life historical situations?)... NO doubt Waters was a great writer, on the whole...
Gabriel on the other hand... he shines in many songs through his 6 Genesis albums... but he is incredibly farfetched in others (specially in the Lamb...) and very cold and to-the-chase in some (The Return of the Giant Hogweed, for example, though I think is a GREAT SONG). Yet, he has proven to be an excelent writer with SUPPER'S READY and his great epics.
Peter Hammil, maybe? Too cold?
Jon Anderson? Ian Anderson? Yes and Tull are not THAT recognized by their lyrics (arguable with, for instance, Heart of the Sunrise - Yes, and the album Close to the Edge - Yes)
I'd want to add another writer, who is well known in Argentina and maybe somebody's heard of him... Charly Garcia from La Maquina de Hacer Pajaros...