It's always a difficult one comparing players from different era's but De Gea is a level above anyone I have seen before. I loved van der Sar but he had an incredible defense in front of him. The same could probably be said for Schmeichel. De Gea has been a one man band for most of his United career.
In terms of the modern day comparisons, Neuer seems to get all the acclaim as the 'world's best' in teams of the year at these FIFA/UEFA conferences but De Gea is on another level for me. What works against him in terms of acclaim is that he plays for us! If he was Madrid's keeper I'm sure he would be widely proclaimed as the best out there.