Ok so, this one might be a bit controversial but I really hate Xander.
When I first watched Buffy, I thought he was pretty funny and actually liked him. He provided some much needed comic relief throughout the show. But that was until he left Anya at the altar. The second he did that I hated him. Anya is such a little cute fluffy cuddly bunny (get it....) and she was just trying to find her way in the world. She fell in love with him, and they were so cute together. She was ready to live out the rest of her life with him, and he's the one that proposed so he should have been ready for it too. He left her heartbroken for literally no reason except for his own selfish doubts. So basically, that's when I started hating him. Imma just look at all the seasons and rant about why I hate him in each one.
In Season 1, he was absolutely obsessed with Buffy. Like literally had no chill about it and was understandably hurt when she rejected him. But then he continued to be obsessed with her and made her feel guilty and bad for liking Angel over him. He literally made Buffy, the person who saves the world weekly, feel bad about her feelings. All Buffy needed was support and comfort and he was too wrapped up in his own feelings to care about anyone else's. Man he annoyed me so much rewatching it, coz it was obvious Willow liked him and he didn't pay any attention to her, rather used her for help from their first interaction. But Willow didn't guilt him about it, she quietly harbored her feelings and tried to move on. Literally from S1 rewatch I could see how selfish he was. He basically used Willow as his rebound for Buffy, and so she rightly rejected him, but of course he was too wrapped up in his own feelings to see why Willow made the right call.
Then in S2, he continued to be obsessed with Buffy. Honestly, he was super annoying when he blamed her for everyone being kidnapped. Like she obviously was suffering from PTSD after dying and he was just so up himself that he couldn't see that, and once again guilted her. He continued teh trend he had of falling for the monster of the week because apparently everyone but him can tell somethings off about these people. Then he started seeing Cordelia, and honestly their relationship was super amazing. It was pretty cute and they had chemistry. Cordelia, who was known for being selfish, broke it off when people found out and so Xander, being even more selfish (although apparently it's not one of his personality traits - yeah right) decides to cast a love spell. Admittedly he did reject Buffy who threw herself at him which was the only good thing he had done in a while. Then, when Angel lost his soul is when he really got going. He basically told Buffy, a teenage girl completely heartbroken, I told you so... Like he can't put aside his pride (which I'm surprised he has any considering how insecure he is) for 2 minutes to be there for his *friend*. Then, he didn't want Angel to get his soul back, coz apparently stopping the murdering guy by making him good didn't validate him enough, it was better to kill him. And once Willow told him to tell Buffy basically to stall Angel, he said go kill him. I honestly blame Xander for a lot of it, like maybe Buffy could have stalled him, made it her number 1 priority to make sure he can get his soul, but instead she acted more rashly and basically was forced to kill him coz she saw no other solution. It literally left her dead to have to kill him, coz SURPRISE XANDER WILLOW ACTUALLY BROUGHT HIS SOUL BACK SO BUFFY HAD TO KILL ANGEL WITH A SOUL... not that he really cares, coz he's just glad Angel's gone.
Then, in S3, and honestly I'm hating all of them for this but I'm ranting about Xander atm, he hated her for leaving. It was his fault that Buffy had to kill the love of her life, and he dared to blame her for everything happening. Like she needed some time to collect herself, she had been expelled, her mother kicked her out (well not intentionally but her phrasing did make it seem like Buffy wasn't welcome) and Xander (and the rest of them) couldn't understand coz they were too busy being mad at her for leaving. Like honestly, I'm siding with Buffy coz she went through MAJOR trauma and then she came back to everyone she loves blaming her. Not fair. And to top it off - he called her selfish... talk about irony. He also hated on Buffy for helping Angel get better, even though he had a soul, and went and literally tried to isolate her from her friends. Honestly, he would have made a great villain the way he belittles Buffy. Later, he had a good thing going with Cordelia, but decided to ruin it by going for Willow, who had had a crush on him for so long. Like this guy is everything I hate. He did do a really sweet thing in buying Cordelia the dress, and saved the town, but he literally is so selfish all the time so these acts do not cancel that out.
Then in S4, I kinda felt bad for him. He had his huge inferiority complex (which I don't get coz he had friends he should go talk to them - but pride stopping him), adn the fact he didn't get to college, had to move in in his parents' basement and all the odd jobs. But then Anya made him feel important - building up the hate I have for him for breaking her and belittling her constantly.
The rest: He continually belittled everyone, which makes no sense because he was so insecure and none of his friends made fun of him as much as he did them. He summoned a demon, which killed multiple people (coz he is that caring about others that he summons demons to help sort out his own problems). Then, straight from his page " (Xander) realized that Buffy had not told them about her relationship with Spike because she knew that Xander would react as he had" - basically saying that Xander is super judgemental and does not take anyone else's feelings into account. He broke Anya's heart because he's too much of an insecure baby to try and work through things, helped gang up AGAINST BUFFY (honestly hated everyone for this too because Buffy is super smart and would 100% have led them to victory much earlier had they let her).
Overall, I just find him really selfish and up himself, and a lot of his comedy comes from belittling others, which is really rude. Also, he doesn't take any blame (except for a few occasions) and tends to blame everyone around him