So, unsurprisingly, it's Xander.
For his proprietary and intrusive attitude with Buffy. For his judgmental and entitled opinion spewing on the decisions she made about her personal life .For his unbending and unrelenting snark about the guys she chose to spend time with, for his naked jealousy that made him such a bad friend. For the way he rallied the Scoobies to brow-beat her for things he didn’t bother to understand (Innocence, Dead Man’s Party, Revelations), for doing all that in front of an audience of her peers so she would be “properly” cowed and shamed.
All that because she didn’t pick him.
For only wanting Willow when she fell for someone else. For cheating on Cordelia and causing an accident that could have killed her. For refusing to feel bad about cheating on her and causing that accident because she wouldn’t make things easy on him by forgiving him. For reverting back to jerk-mode once they were broken up and refusing to understand why she might not want to hold hands anymore. For still thinking of her as “his” Cordy as late as the end of their senior year.
For blackmailing Amy into causing a spell which would make Cordelia fall horribly in love with him so he could turn around and dump her to assuage his own fucking ego.
For daring to judge and argue with Buffy for going after Anya after the shit storm of guilt he leveled at her for not being able to kill Angelus.
For hating Angel for “getting there first.”
Xander has some good points. He’s saved Buffy’s life and he’ll always have her back. But his flaws are too glaring and, for me, they eclipse his good traits.