We really need to define peak. I will define it a different way in response to Wilander's quote. For this instance, I'll define peak as the best play over an entire tournament. I.e, whose play for one tournament looked the most unbeatable? I'm going with Nadal, when factoring in all 3 surfaces.
Clay: Nadal is a beast on clay and nobody in history on any surface comes close. Before injuries slowed Nadal down a few steps(especially prior to 2011 when Fed played him tough), Nadal was unplayable on clay.
Grass: Nadal peaked incredibly high on grass. He beat Fed at the 2008 Wimbledon and he played 2007 Fed incredibly tough. Looking back, this 2008 loss was the most painful as a fan. I really wanted to see Fed break Borg's record of 5 straight Wimbledon titles. Dominance to me is winning titles consecutively. I love consecutive records. I really wanted Fed to own this one. But Nadal was too good this day. Now granted, Fed peaked higher than Nadal on grass. I think Fed from 2005 beats 2008 Nadal at Wimbledon. But Nadal's peak on grass was incredibly high.
Hard Court: Nadal at the 2010 USO was insane. This is the only time that I saw Nadal hit several serves in the 130's. The announcers mentioned that Nadal added 10 mph to his average 1st serve speed during this tournament. This Nadal at the USO was unbeatable. Only a peak Djoker(Djoker's play at the 2010 USO and 2011 USO were similar and both beat Fed in 5 sets) could take a set off of this Nadal. Nadal's serves fell back down to earth in 2011.
Now clearly, sustained peak/dominance is the key. So I am not saying that Nadal has been anywhere near Fed's universe on grass, or near Djoker on hard courts. What I'm saying is that Nadal had the highest level for a tourney ever on clay by a long shot, close at the Wimby slow grass, and the best tourney at the 2010 USO.