I voted Willow/Kennedy as the worst couple.
A lot of the couple weirdness can be directly blamed on Angel getting a series of his own. Willow/Xander was just a quick easy way to breakup Cordy/Xander & have her go to LA. Buffy/Spike was nothing more then a replacement for Buffy/Angel, right down to the part where Spike gets his soul.
The only couples I enjoyed on the series were Anya/Xander, Cordy/Xander & Willow/Tara all the others sucked to some degree IMO.