Defenders
It also helped that he had Ronny Johnson and Jaap Stam (who IMO, was the best defender in the World, when he was with us).
VDS had Rio and Vidic.
For me, DDG is the best, only because he has to play with an average defence in front of him.
Attackers
When PS was here we had Cole, Solkjaer, Sherringham, Yorke, Beckham, Giggs as our attacking line up.
When VDS was here, we had the likes of Rooney, Ronaldo, Tevez.
When a GK knows that we can score 3+ goals against even the best teams in the World (remember scoring 3 goals against Juventus, in 1999, who at the time were regarded as the best team in the World?), it reduces pressure on him.
With DDG - he knows that if he lets 2 goals in, we've lost. So the pressure to keep a clean sheet is immense. Pressure usually leads to errors, but that doesn't seem to be the case with De Gea.
So, who is the best?
DDG, by a tiny amount, for reasons listed above.