I think Xander and Cordelia were an interesting pairing, the on/off nature of their relationship and constant sparring made for great television.
Xander and Anya started out as a similarly mismatched couple, then became more believable as their obvious feelings for each other became apparent. But, unfortunately, they seemed to lose the spark a little in season 5 when Anya became clingy and rather annoying, in my opinion.
Xander and Willow, as briefly as they were together, seemed to be the ideal match. I think that they are the perfect example of a couple that just never seem to be on the same page at the same time! Firstly Willow holds a torch for Xander and he pines over Buffy, then when he finally notices her they're both in a relationship. Now, obviously, things have changed so radically that there seems to be no way back!