I guess it all depends on what you like most; decent plot or strong characterisation. Inca Mummy Girl has an awful plot, truly terrible in fact, but all the characterisation is spot on- its even got some good jokes.
'Teacher's Pet' falls into the same catagory imo, plus almost all the S1 episodes have a kitch edge that tends to soften their awfulness, and the writers are still finding their feet.
The later episodes from mid-S2 don't really have that excuse which makes them worse imo, there's no excuses for some of them.
And I have to say after recently watching 'Go Fish' it isn't really as awful as I had remembered, it just kinda falls apart towards the end.
But 'Dead Man's Party', 'Beer Bad', 'Dead Things', and 'As You Were' are all strong contenders- containing lame plots, unpleasent scenes, lame jokes and poor characterisation.
I actually recently discovered that Tracey Forbes only wrote three Buffy episodes- all of them in Season 4- they were 'Beer Bad', 'Something Blue' and 'Where The Wild Things Are', two out of three pretty bad episodes might explain why she didn't write any more scripts after.