I know this seems like a question that's been done, but this isn't about who commited the worst betrayal, it's about who said or did something that most deserved a response.
Even though I personally think the worst betrayal was from Willow I definetly think what Anya said was the most annoying and the one that most deserved a response.

Firstly the way she says 'you didn't earn it' as if fighting for your life every day and having the weight of the world on your shoulders is some kind of treat, and secondly Buffy killed the love of her life, saved them all many times and even died twice to save the world...If that's not 'earning it' I don't know what is, and I wish Buffy had pointed that out to Anya.

Simply because i hate her and she always tryed to take over and include herself into the scoobies.

i'd be more than happy to do the telling off of dawn for buffy...*cracks knuckles menacingly*

Dawn pissed me of the most.telling buffy to leave.its also buffy's house and for pits sake buffy died for her.
Does she remember that at all.

Because Buffy had previously sacrificed herself for Dawn, because Buffy is Dawn's legal guardian so a telling off would make more sense, and because it'd been Dawn who'd thrown her out of the house even though that house should be either jointly theirs or more likely Buffy's.