When I say Cordelia Chase is the best character from Buffy the Vampire Slayer I don't mean she is the most fun or entertaining. Some may think she is but I mean she is the most morally pure, heroic character on the show. Genuinely the best.
Cordelia Chase is presented as the epitome of vice. She is vain, prideful, greedy, and lustful. Despite knowing Buffy and Angel have a thing going on she covets him for herself. Her words are often dripping with venom and directed at innocent people. The Scoobie Gang all hate her and it is mutual. These aren't just superficial flaws hiding the real depth of Cordelia Chase. This is who she is to the very core. There is an episode when Buffy develops the ability to hear the thoughts of others. With Cordelia, alone, her thoughts are just the things she says a second before she says them. There is no hidden depth, at least not at first. She doesn't hide things.
And, yet, at the end of Season 1 Cordelia risks her life to rush back and save Willow and Ms. Calender and helps them fight off vampires. Through out season 2 she is always there if they need a ride or help researching. This doesn't lead to a close relationship with the group. It still steeped in mutual dislike. But the fact is that Cordelia, almost alone among Sunnydale students, cannot ignore that she is surrounded by supernatural evil. Although many students have brushes with the supernatural and are saved by Buffy, most are perfectly willing to pretend nothing is happening. Cordelia Chase is not that type of person. She is honest to a fault. That honesty makes her say the worst things she thinks but it also prevents her from lying to herself.
Buffy, Giles, and, to a lesser extent, Angel don't have a choice. They are chosen. They have destinies. It is made clear for all of them that they wouldn't lead this life if they weren't chosen and all have tried to fight it (ask the Ripper). Xander and Willow were outcasts who find purpose and family in fighting the undead. Willow also becomes increasingly seduced by magic from Season 2 on. Even Oz doesn't get involved until he falls for Willow. All of them get something for their troubles. At least until things get much darker later on.
By contrast, Cordelia only loses her friends and everything she values (genuinely values) by choosing to side with Buffy and crew time and time again. No deep bond is formed between her and most of the crew aside from Xander. And she ends up falling for Xander (figuratively and, ultimately, literally) because she has been helping solve mysteries and driving him around not vice versa. Once Xander has broken her heart she still stays and helps fight on her graduation day. Even when she blithely wishes Buffy had never come into town and accidentally plunges Sunnydale into an apocalyptic hellscape, she quickly acts to try to spread the word and fix things.
This character trait is further explored in a less subtle way in Angel. But the fact is she is one of the few people in the entire Buffy universe who fights evil only because the only way to not do so is to pretend the evil doesn't exist. Only because to do otherwise would mean pretending everything is okay and that isn't who she is. Prior to Angel, she doesn't really gain friends or a sense of family from it. Prior to later falling for Xander, she doesn't have a crush or love interest that ties her to the group. Unlike many of the main protagonists, she doesn't have any grand destiny that she must complete. She is just a normal person like every other normal person in that high school. Except she refuses to be blind and refuses to let other people fight her battles for her alone. Because quite often the people who don't bother to hide their vices are the most virtuous ones of all.