I don't think it was even in the top 5.
Tara finding out that Willow was erasing her memory and then finding out that she continued to do it after Tara told her that it was violating her brain and after we heard her sing about how "they'll be nothing left of me" and "I can't adjust to this disgust." It was months after she was trapped in a severe psychosis by Glorificus. Her own girlfriend/lover and person she trusted most in the world messing with her sanity. And continuing to do it after she threatened to break up with her.
Faith switching bodies with Buffy against Buffy's will. Not caring that Buffy was drugged, tied up, chained up, and could've been raped, tortured, or murdered by the WC hitmen. That Faith sexually violated Buffy in the bathtub. That Faith tricked Riley into making love to her while she was in Buffy's body. "How do you want me? What nasty things have you been dying to try out on this body? Am I a bad girl? Do you want to hurt me?" She took Buffy's agency away from her, became sexually intimate with her in a way that can't get more invasive (knowing what Buffy looks like naked, feels like, actually controlling Buffy's body's orgasms, knowing how Buffy's body feels when it has sex), and let someone else have sex with Buffy's body without Buffy's consent.
Willow finding out Xander was fooling around with Cordelia. It seems like nothing but it was at a time when Willow had nearly zero confidence. Her feeling like a nobody, a loser, a nerd, inferior and weak, is what drives her the entire series, it is why she becomes addicted and out of control with magick. She had been best friends with Xander since kindergarten, he was her closest friend, and she was in love with him and even secretly planned their wedding. And there he was with the physically prettiest and most popular girl in school, the girl that had been mean to her for years to the point that they created a We Hate Cordelia club. "You'd rather be with someone you hate than to be with me."
Joyce in Gingerbread. She was told by two spirits to hurt the bad witches that killed them. She was not mind controlled. All that it took to get some common sense into the mob was to show them what the kids really looked like; an ugly giant demon. And yet she chlorophorms her only child, even commented on how emotionally easy it was to do so. She lets her child get tied to a stake. She actually gets a burning torch and lights the kindling surrounding her daughter on fire. She was going to burn Buffy to death, murder her 17 year old daughter. She knew that Buffy spent years risking her life to save the innocent and destroy the forces of darkness. She tried to kill her because she couldn't accept that Buffy wasn't what she wanted her to be. "What I wanted was a happy normal daughter. Instead I got a Slayer."
Xander and the infamous lie. He knew that it would lead to either Buffy having to kill the only man she had ever loved or sending that man to Hell, possibly for eternity. His jealousy and pettiness towards Angel ("I hated Angel long before you all jumped on the bandwagon so I think I deserve something for not saying I told you so") nearly destroyed Buffy (the grief and guilt) and meant 100 years of torture for Angel.