Did you guess that my choice for this one would be Episode 6x07: Once More With Feeling?
I enjoy this one, which starts with Buffy singing while out vampire hunting in a graveyard. As usual, there is an explanation for why every character is breaking into song constantly (Lorne from Angel would be on overload reading peoples' souls), and its because a demon has been summoned who gives humans and urge to express themselves through song.
On the face of it, its a comedy, pure and simple, but it has moments of pure sadness, as only Joss Whedon can write them. Because everyone is forced to belt out exactly what's on their mind in song, Buffy reveals to the others (after telling Spike, "they must never know") that when Willow resurrected her, she was torn away from Heaven, where she would have been happy for eternity (fans can only hope that Buffy would still go to heaven after her eventual dying for good).
Also, this episode sets off an arc affecting Willow's relationship with Tara that lasts for most of the season, when a particularly manipulative spell is on Willow's part is discovered.
So yes, despite the laughs, there's a slightly bitter centre to the episode, which also sets up "Spuffy" as a couple as well.