In my opinion (and it's based on relatively limited experience(!)) I'm not sure if it's necessary to wow and impress a new student with an impressive piece. I think sometimes it can be a bit intimidating to a student at a time when they need to be given confidence and not feel that being 'good' is all that important (since they're likely to be quite 'bad' for a while being an absolute beginner).
Also, I've always thought a good starter would be the Peter Gunn theme from the Blues brothers. I had this idea of doing a lesson called
"It's as easy as A, B, C, (and D)"
Because the notes in it are A, B, C and D.
http://www.last.fm/music/The+Blues+B...ter+Gunn+Theme
actually I just listened to it and there's a little hammer on.. So technically there's a C# too. Oh well I'm sure a beginner could learn the easy version...