Also my favourite episode of all time: Episode 4x10: Hush.
An episode that could have been filler were it not for the introduction of Tara Maclay, Willow's first girlfriend. This one is great for its main gimmick: A (mostly) silent episode, as the Gentlemen descend upon Sunnydale, and steal everyone's voices (because a human voice will kill them), all so they can prey upon the inhabitants.
So, for a huge chunk of the episode, there is no dialogue, save for Professor Maggie Walsh using a Stephen Hawking-like voice programme to communicate with the initiative.
I love this episode also for the really creepy, horror-like atmosphere, right from the start, even before the Gentlemen show up, and also for some of its comedy value (Buffy miming using a stake, and appearing to mime something else is hilarious).
An innocuous-sounding episode that turned out to be a classic.