I have actually been thinking about this, while rewatching the series recently...I have some friend who know I'm obsessed with Buffy, and refuse to give it a try because it's about "a girl who fights people in rubber masks"...I think some people see that as being all Buffy is about, plus, I think the title really throws people off. I know I won't be able to convince them anytime soon to watch (though I recently got two people at work to watch the whole series and start reading the comics, but they are more open minded)...but in my opinion, Passion is the perfect episode to start someone off with.
1.) It's not overly supernatural...not too much vamp stuff, and no demons.
2.) Has some great laugh moments (Cordy worrying about the vamps being able to get in her car whenever they want).
3.)And amazing drama (need I mention it).
I think it's the perfect episode to show that the show can go all sorts of places, and some weeks be crazier than the next.
My other choices would be Bad Girls and The Body!! If they aren't hooked by then, I don't think they'll ever be haha.