Dude you've got to be joking! Illgner?!! Zenga?!! Nostalgia aside, those 2 were barely above average even in their own era.
Buffon is indeed legendary. When he was at his peak, he was close to what De Gea is now, but De Gea wins because he definitely is more modern and more athletic. Buffon easily wins for his leadership, presence and achievements.
Nuer is one hell of a sweeper/keeper, but reflexes? Please! De Gea wins. Nuer is indeed the best keeper in history when it comes to being an "all around" player, but for pure goalkeeping, De Gea is #1.
Iker can't be considered because of his strange prolonged downward trend. I strongly believe depression took over this guy's life and never left.
Pfaff and Preudhomme were excellent keepers, but they didn't face the level of football De Gea is facing and according to most goalkeepers, the new balls are much harder to anticipate than older ones so again De Gea wins.