The Body for sure. In terms of directing, writing, acting, editing, everything, it was excellent. There are all sorts of interesting and subtle details in this episode that make it a masterpiece. I rewatched it this week and noticed this tidbit. From the shooting script:
TEACHER
Okay, remember, we're not drawing the object, we're drawing the negative space around the object. We just draw the edges of the figure, and then give me a sense of the spaces around, the space in between.
[...]
As Kevin and more kids move toward the glass to see the girl on the floor, whom now we cannot hear -- in fact there is silence entire -- the camera forward, loses them, turning to focus on Dawn's drawing: the outline of a figure, but not the figure itself. Negative space.
I like how the episode is not about the body - Joyce - but rather what surrounds the body, meaning the negative space - the scoobies. And the outline of the figure that Dawn draws looks like her mom lying dead on the couch. It's such a cool detail that shows how well thought out this episode was. It's one of the few Buffy episodes that's perfect from start to finish.
The Gift is a good episode but only the ending is great. Everything else is unremarkable in my opinion. And the ending tries too much to be epic, it becomes cheesy.