There's not a specific episode, but some of the stand-alones in Battlestar Galactica's third season. I liked some of them well enough but overall they didn't really serve much purpose for furthering the bigger story that was being told, which for a series like BSG everything was fluid in its storytelling so it seemed kind of off when they did one of those here and there. I believe it was the network that pushed for those stand-alones until Ron Moore took control again and decided he was going to do things his way.
For Buffy the Vampire Slayer there were many episodes I didn't favor, most of them were in the latter half of the series, especially in the sixth season because that was my least favorite season ever. So it's really hard to say. I think "Where The Wild Things Are" in season four was just that random wtf kind of episode where it was just so horribly bad, it's that "what were they thinking?" kind of an episode. The only redeeming quality of it was having Anthony Stewart Head singing with his guitar in that one scene, because him singing is never a bad thing. But yeah, I think even though I loved BTVS it had quite a handful of cringeworthy episodes which, for a series running as long as that of course it's gonna have its bad eggs of either episodes or seasons in general. It's just the inevitable.
And for the sake of it, in Supernatural it's mostly the filler episodes in these last two seasons which they should have minimized greatly when dealing with a larger than life apocalyptic plot. As for specifics, definitely "I Know What You Did Last Summer" and "Heaven and Hell", which I think everyone knows the obvious reasons why those episodes sucked major. DNW ever again, Kripke.
Masterlist of days here.