Cander worked well when Cordelia was on Buffy but realistically that relationship would not have lasted long term. Cordy and Xander are just two completely different personalities with two completely different outlooks on life. It might've worked if it had been AtS Cordy since she kind of had a fall from grace and money problems and was therefore more humble than she was before, but BtVS Cordy would not have dated Xander past high school.
Cuffy wouldn't work because neither Buffy nor Cordelia really like each other. They might feel sorry for the other every so often, but they wouldn't go out of their way to spend time with each other.
Cesley had there mini romance in season 3 and it fizzled out extremely quickly. They wouldn't have been able to make it work, Cordelia would've gotten bored. Wesley also matured faster than Cordy did, he would've started to get annoyed with her pettiness.
Coyle was cut short before its time so it's difficult to say whether it would've worked long term or not.
Croo was not a good relationship for Groo. He loved Cordy more than she loved him and he did the right thing by getting out of it when he did.
Cangel shouldn't have happened. He is Buffy's man and always will be. Why would Cordy allow herself to be second best when she knows Angel's heart belongs to Buffy?
Cordelinor was an abomination that never should've happened. It was wrong on so many levels and completely destroyed Cordy's character.