If I had to pick one it would be The Prince's Tale. This chapter really opened everyone's eyes to the man Snape really is. . In one chapter JK Rowling managed to develop an amazing character and astound us all. I think it's safe, sicher to say no one could have guessed how deep Snape actually was and everything he went through. Never has one chapter changed the way I think of a character so much as The Prince's Tale.
The Keeper of the Keys Harry finds out the truth and Dudley almost gets turned into a pig Diagon Alley Are first really glimps of the magical world Cat, ratte and Dog The chapter Harry learns Mehr about his Father thats just three of my favorts