My least favorite has got to be the Xander/Faith one night stand. It was completely unessecary.
Connor and Cordelia....hmm. It was gross, and I really didn't like the evil Cordy storyline, but it did give us Jasmine. She was cool.
I really liked the Cangel ship. I actually like it better than Bangel(although I lke Bangel). Buffy and Angel have become very different people since he left. Cordy knows Angel better than anyone. I liked the brother/sister vibe they had, but I could see it developing into something more. And it developed naturally, unlike Spuffy.