Rachel does show herself to be pretty horrible for a good length of the show's run, honestly. Monica gets weird later on with her stupid competitiveness, which to me would be something borderline friend-fireable. Phoebe is awful for a good long time but kind of bounces back in later seasons. Joey and Chandler are generally good guys who makes mistakes, unless you consider that Joey is pretty much a user of women, but in a world where their show overlapped with Family Guy being on the air he seems pretty tame by comparison. And Ross... Ross was just made into an idiot more than anything, his character was assassinated from the second the Emily marriage shattered.
But thinking about it, Rachel really is horrible.
- Leaves Harry at the alter for basically no reason. Later sleeps with him again, despite the fact that he is with her best friend.
- Is completely resentful and hateful towards Julie, who is completely innocent in the entire situation and knows nothing of Rachel's feelings which, lest we forget, Ross doesn't know about either
- Goes to ruin Ross's wedding
- Lies to Ross's face about being mad at him over being left at the airport, when OBVIOUSLY Ross would go after his recently jilted wife
- Exchanges every gift she ever receives without even checking with the original purchaser whether they're okay with that
That's just off the top of my head, I could probably think of more.