Saryann
Wasn't real couple, but would have been nice and funny...

Spike/Willow blaugh

And about real pairings:
Buffy/Angel
Xander/Willow
Willow/Oz
Willow/Tara (at 5th-6th seasons)
Spike/Drusilla


If we're talking about couples that didn't actually happen, I would've loved Spike and Xander... That show needed some man love. ^_^ Plus, I do have my suspicions about those two...