Oh dear. I'm getting tired of these least favourite questions because they are so negative and as Andy pointed out the answer usually involves S7 and/or Kennedy.
Well, of course, I didn't like Kennedy and Willow because of Tara and because she was pushy but I kind of liked her in "The Killer in Me" so I'm not choosing them. I didn't like the way Buffy and Riley's relationship ended but I liked their awkward romancing so it's not them (though I suppose him being a TA and her being a student was a bit iffy). Angel and Buffy were crucial to S2 so it couldn't be least favourite. Stalker Spike should have been appalling but he was so pathetic and she kept hitting him so that made it - a bit annoying - but still not least favourite.
I think it has to be Buffy and Parker. The boy screamed creep as soon as he turned up. Come to think of it so did Owen and Scott.
I think Wesley and Cordelia would vote for themselves.