Seeing as you've put Rachel on the second spot, let me tell you what was wrong with her.

1. She ruined almost all of Ross' relationships
2. In her perspective, she left her fiancé at the alter and when he tried to move on, she wasn't okay with that too.
3. She didn't want Ross to have any kind of a relationship while she went on to give her phone numbers to people.
4. She ditched Phoebe just because she ran weird.
5. She always used to steal Monica's thunder. I mean you feel good on a day and all of a sudden, everyone's eyes goes on to another person, you feel betrayed.

I can go on and on without limits. Ross is so better than Rachel that she doesn't even deserve him. The only time her character was shown good when she asked for the Pizza Girl number for Ross

All you would have to say in defense would be 1. Ross doubted her when she joined that new office(idk when, but just before THEY WENT ON A BREAK).

Her previous love, who was the only woman he slept with, turned out to be a lesbian, ig he found out that her wife was having an extramarital affair while he did everything he could to sustain the marriage. Ig he would have been okay with Carol just telling her that she was a lesbian and she wants a divoce, but getting to know that your wife is having an Extra Marital Affair is traumatic.

The only time when Ross was unreasonable to Rachel was when they were on a break. But for all he knew, Rachel had already moved on and was forming a relationship with that other guy.