Let's see:
Teacher's Pet - Just awful, even for an early season 1 episode.
The Pack - Even if only for the ridiculous cackles and the longest, most pointless slo-mo walking scene ever.
I, Robot...You, Jane - One of the few episodes that gives me legitimate second-hand embarrassment.
Some Assembly Required - Boring.
Inca Mummy Girl - It's just...well, stupid.
Reptile Boy - Willow's scenes are the only redeeming qualities of this episode.
Bad Eggs - Giant bugs creep me out so much! Eugh!
Go Fish - "Dude, what is that foulness?!" Do I need to say anything else?
Beer Bad - Listen close to every Buffy fan, and hear that cringing soooound...
Where the Wild Things Are - I can barely watch this one.
Doublemeat Palace - Two words. Penis monster. Spike does look ridiculously hot in that scene where he visits Buffy at work, though.
I'd include some episodes from late season 7, but really, I can't remember them that well. After the Potentials come, it seems like one big, fairly lousy episode with a couple of decent moments sprinkled in.
That does seem like a lot of episodes now that I look back at it, but really, even they were much better than most TV shows out there. Besides, the great episodes vastly outnumber them.