Depends on my mood:
If I think I'm awesome I'll attempt Empire State of Mind - it never ends well.
If I'm feeling lonely - All by myself by Sheryl Crow
If I'm in a bouncy mood - Hold On by Wilson Phillips
If I'm with primary school friends - Wannabe by the Spice Girls, cos I totally ace the "Now we've got Em in the place" part
If I want to annoy my mum (while playing singstar at her house) - Anything by ABBA
If I want to beat my mum - I've Been to Paradise by Charlene.
There's a karaoke song for every occasion!