Oh! My least favorite is Flooded. I remember I got it confused with Life Serial (which I love haha) and I watched it to the end. Then I thought, "Oh crap. That wasn't the right one." First Date is also kind of eh...but Flooded is definitely my least favorite.
Beer Bad
Dead Things
Go Fish
Inca Mummy Girl
Where The Wild Things Are
As You Were
Doublemeat Palace
(pretty much most of season 6)
But ... how could Flooded be your least favorite of ALL???? Even if you're not a raging, obsessive Giles fan *cough*like me*cough* still the reunion scenes between Giles and Buffy are epic, and crucial to the development of the story; and the confrontation between Giles and Willow is one of the best (and best-played) dramatic moments in the history of the series, plus it sets up "Grave."