Hmm, this is a very good question.
Page is responsible for their riffs, and founding the band in the first place from the ruins of the Yardbirds. But would it be the same without Plant's voice? Bonham's drumming? JPJ's bass? It's a very hard question.
I would lean towards Page, however, as he did come up with the idea originally, and without him it wouldn't really matter how vital the others were.