Greater is an easy question IMO. It's Wawrinka. But I don't think he's much better, if at all. Simply because even the Stanimal of 2013+ never would've won slams in Federer's prime years because of a bad match up. And he wouldn't have a RG either. He only won one of those when a relatively old Nadal was in a slump which never happened to young Nadal. Plus, even if by a miracle the young Nadal stumbled ala 2015-16, a younger Federer would still be like an 80/20 favourite on clay.
In hindsight, the Wimby 09 Final loss hurts Roddick a lot here. With a 2 slam gap it's basically impossible for Roddick to win the greatness stakes, but a second slam and a win over Federer on grass and still pretty much in his prime on top of it would've tipped the scales quite a lot.