So, my favourite Season One episode is - a guilty pleasure one...
Episode 1X10: Nightmares
Before Anya or Halfrek was causing chaos with vengeance demon powers, another character had the power to create bizarre alternate realities, all from peoples' nightmares.
So, a comatose boy is somehow causing nightmares to manifest themselves; Xander finds himself in his underwear at school, one of the comedy highlights of the episode, another character is attacked by spiders that appear out of a book.
More seriously, Buffy's father shows up and tells her that it was her fault he and Joyce split up because of her behaviour, and then says that he doesn't want to hang out any more, before storming off. It's quite a heartbreaking moment, so it's good that it turns out to just be another alternate reality that Buffy is in.
After this, the episode turns completely surreal; Xander is chased by a clown, Willow finds herself forced to sing Madame Butterfly, Buffy vamps out and giant insects attack sunnydale.
I tend to enjoy crazy episodes, which is the main reason I like this one.