Ross was very unpleasant, but he also had a shit deal handed to him when he left college.
He married his college sweetheart, only to get dumped when she had an affair with her best friend from the gym. They then raise his son together and he barely gets a look in because he's not with Carol anymore.
But none of that excuses how shit he was to Rachel. He didn't like her having male friends, he got insanely jealous when she had sex dreams about Joey and Chandler, he didn't like her being with anyone else (Paolo) even though it was perfectly okay for him to be with Julie.
He didn't like her job, he came to events with her and embarrassed her by falling asleep in front of all her co-workers. He showed up at her office to "mark his territory". He cheated on her with Chloe (they weren't on a break, she wanted to get back with him and she said okay, but got jealous of her speaking to Mark and went behind her back).
Even when they had Emma, he didn't like them having a male nanny, got jealous when she met another man at a bar (even though they weren't together) and didn't even like her living with Joey.
I realise I've thought about this way too much, but loads of my friends used to want a relationship like Ross and Rachel and failed to see how badly he treated her.
Give me Chandler any day 