The one thing that sticks in my mind with De Gea is the unbelievable amount of stick he got from all corners of the media during that first season. Everyone was desperate for him to fail. He was targeted as a weakness in almost every game. Always blocked off at corners, taking ridiculous pot shots from every angle. The Mirror kept calling him "De Howler" among other things. Many were saying he was a waste of money, Lindegaard should be the #1 for the remainder of the season and we were tipped to dip back into the market for another #1.
He definitely had weaknesses, but how quickly he improved on them and acclimatised to the physical nature of the league, all whilst ignoring pelters he was getting from everywhere, was quite astonishing. For me, he puts to bed the much-used remark of "not suited to the Premier League" that is used an excuse to not want a player, or to excuse poor performances of a new player. De Gea probably wasn't suited to the Premier League, but within a year or so, he made sure he was.
Now he is the undisputed best in the league, with an abyss of nothingness behind him, the gap between him and everyone else is that significant. He is currently sticking two fingers up at everyone who targeted him, doubted him, egged on his apparent certain failure. It puts De Gea at the top of the pile for me personally. I'm sure Schmeichel and Van der Sar went through periods of vast improvement elsewhere, but to see it from De Gea first hand as a supporter was magic. Schmeichel was my favourite player growing up, I sulked for days when I found out he was leaving, he was phenomenal. Still, De Gea is the best of the three IMO.