Oooh... this is hard. I want to get my mom addicted, so I'm making this list with her in mind. I tend to like the funny episodes, but my mom hates that stuff. So I'm trying to go for episodes with more emotion or even melodrama and more complex monsters. It's funny, because I have to shy away from picking my favorite episodes in each season.
Season 1: Prophecy Girl
Season 2: Passion
Season 3: Amends
Season 4: New Moon Rising
Season 5: Fool for Love (I would pick "The Body," but my Grandma died recently and my mom is still pretty shaken up about it. I don't think she'd make it through to the end of that episode.)
Season 6: Entropy
Season 7: Touched