Cordy and Angel for sure!!!! I think Angelus said it best in 'Soulless'.
Angelus: And now my boy's in love. All hearts and flowers. But, doesn't it freak you out that she used to change your diapers? I mean, when you think about it, the first woman you boned is the closest thing you've ever had to a mother. Doing your mum and trying to kill your dad. Hmmm. There should be a play.
Cordy and Angel (not that they ever really got together) were so sweet cos it wasn't just a rushed relationship thing, they really had grown to love each other.