Well, while I think that it would be difficult to appreciate the emotional aspects of the show without having watched it from the start ( There would be no personal connections to the characters, etc.), I think that I would have to either go with Prophecy Girl, or agree with Magui and say Innocence.
Prophecy Girl really throes the viewer into Buffy's world. One get's to experience to pure hell of high school and growing up, having to take reponsibility for one's self and ations. Then also there's the sentimental friendship aspect. There are some nice scenes between Buffy and Willow, Buffy and Xander and WIllow and Xander. I think that this is the first episode of the show to really illustrate how amazing it is. (aside from the horrid use of the theme song).
Innocence has all the drama and heartache of adolescence and love. Love is that one universal thing, everyone can relate to and sympathize with it. No matter what your culture and the way things are done, there's always love. It also has all the trappings of high school, friendship and growing up and facing the not always plesant reality... Also, this episode completely sucks the viewer in.