I have often thought about my favourite Buffy episodes, and I always go for the 'fun' episodes - generally the ones that have some kind of genre-bending twist to them. Anyway, whilst in the later seasons this is less true, these sorts of episodes tend to not do much to advance the plot. So, here I have begun to compile a list of what I think are the best 'fun' episodes and the best of the more serious 'plot' episodes in each season. If you disagree, or agree, with any of these, feel free to let me know.
The Pack: The first of many episodes in which we see one of our principal cast members put on an alternate personality. The animalistic qualities that Xander and the others take on are verging on the silly, but it's a lot of fun. This episode also gets big points for pushing some major boundaries, and showing us that this show wasn't going to pull any punches when it comes to killing off (apparently) major characters. So, a partial credit in the 'plot' column, as well.
Nightmares: I love the kind of episode in which fantasies or, more frequently in the Buffyverse, fears are brought to life. It is a great way of exploring the characters' psyches and to have fun doing so. Anyway, knife-wielding clown - need I say more?
Honourable mentions to 'The Puppet Show' and 'Out of Mind, Out of Sight'.
Angel: The biggest problem with this episode, in hindsight, is that Darla is killed off both too soon and too cheaply. We don't get enough insight into how significant an act it is for a vampire to kill its sire. Although, to be fair, that is explored a lot more in later episodes of both Buffy and Angel. Anyway, this episode is vital to the Buffyverse, obviously, because it sets up Angel's entire backstory which is key to season two and three of Buffy, and of course for the spin-off. It's a pity that at this point they are doing a lot of telling instead of showing, but the gorgeous period scenes of the later seasons would have been impossible on season one's barebones budget.
Prophecy Girl: It's funny watching season one over again, because The Master is such a weak villain, and has such power over Buffy. That shows good character development, I think, because season seven Buffy would have torn The Master a new one in a heartbeat. Anyway, this episode features the first time we see Buffy save the world (unless you count The Harvest), albeit from the lamest puppet monsters since early Dr Who. Also, Joss Whedon was clever enough to make sure that Buffy noticed the broken table, and deliberately pushed The Master onto it - there's no Deus Ex Machina here.
Honourable mention to 'Welcome to the Hellmouth' - it does a good job of presenting the massive pile of exposition without boring those who saw the film, and without confusing those who didn't.