Great thread idea!
I really like Spike and Dawn's friendship. They understand eachother, both being the underdog and "different" in a lot of ways.
The Mayor and Faith, while dysfunctional...there was real love there, I believe. The look on his face when he sees her lying in a coma...it touched me.
Spike and Angel. These two claim to hate each other, but will save each other without a moment's hesitation. There is so much baggage there, and so much history. There is so much we don't know about these two; their past, how they really feel about each other, what they've been through - but one thing is clear, these two love each other. No doubt.
Willow and Buffy - before everything went to Hell, that is. I love their scene when Willow decides to go to UC Sunnydale. Buffy is so happy. I believe, as is often the case, that these two naturally drifted apart - but that doesn't make the dissolution of their friendship any less sad.
Xander and Buffy, boy do I love these two. He admires her. She's his hero. He loves her like family. He'd risk anything for her. If ever there was a perfect friendship in the 'verse, I think it's this one.
Buffy and Spike. You knew I'd mention them, of course! These two have been to Hell and back together, they've used, abused, and come full-circle. They have been enemies, lovers, and finally, friends. They mutually respect, admire, and care for each other. They accept and embrace each other's flaws, and most importantly, believe in one and other's goodness. He is her "dark place", and she is his light and saving grace.
...and last but not least, Angel and Faith. They have something very rare; a complete and utter understanding of each other.