Bangel.
Cordelia understood angel more than buffy. She humanized him whereas buffy loved him but never saw him beyond his vampire status, albeit with a soul. Angel is way more a vampire with a soul but we never saw that until Ats. Cordelia had much more esteem for angel than buffy. She saw in angel not only a vampire but a person with his feelings, desires and pains and so we felt invested in him, wanted to know him and to be proud of him. Angel needed an unjudjemental person and this person was cordelia, who treated him as her equal. Angel with buffy again... well there isn't any growth for him.
Now you made me sad. And mad. Grrr. Bangel lightning is coming from my eyes.
Buffy never saw his as a person. Never saw him beyond his vampire status? They NO ONE did.
No growth? Well, that was the point. He left, because both him and Buffy needed to grew up. We are all talking about how Buffy was a kid, but he was also. It doesn't matter how many years he has, the thing that matters is that he didn't grew up. They both had to live their life to the fullest, experience new things. And when they finish, when they sense that this is it, they will new they grew up. And we Bangelers belive that they will feel they are meant to be together.
But, Buffy was the first one who saw him as a person. When he returned for Hell, they all saw him as a killing machine, she saw person who feels better. And she herself said, I mean asked, "Can vampire be a good person", when she found out Angel is vampire.