Lucy In The Sky With Diamonds.
No one EVER expects to hear that opening line come from a ukulele. It can actually cause people to spew beverages out of their noses.
Another good one is Simon & Garfunkel's, The Boxer. It shocks people to hear that intro riff, and the subsequent rolling fingerstyle of the song sounds really good on the ukulele.
By the way, both of those songs work better on a soprano...
SHOCK VALUE.