Biggest betrayal in the series?
It has to be Snape's betrayal to Voldemort. Snape was so close to Volemort and was probably his most trusted Death Eater and when I learned that Snape was a double agent I could barely believe it! I was just baffled at how someone so close to him could actually be good. I also chose this because at how good an actor Snape had to be to pull it off. He had to be perfect when pretending to be on Voldemort's side, no one could doubt him, no would could even think for a second Snape was anything but a Death Eater. And the fact that he pulled that off so flawlessly amazes me.
Also, maybe the moment when we are told about Lily and Snape's falling out and what happened. I just can never really understand why Snape called her a "filthy Moodblood". She was defending him and he repaid her by embarrassing her in front of their peers.