What's your favorite song to sing karaoke-style? If you don't have one, why not?
Pretty much anything by the Chili Peppers. Their songs tend to be easy to sing along with, as well as being good party starters. (Think more of the older, funkier stuff when you read the preceeding sentence, not the newer stuff where you have to actually, you know, sing.)
Oh, and I pretty much always lead off with the Beatles version of "Twist and Shout".