What are some good pop songs for an audition?

I'm auditioning as a vocalist for the entertainment at Busch Gardens. It says that they prefer that you sing pop music. I need 16 bars of a slow ballad and 16 bars of something up-tempo. I was thinking maybe some Christina Aguilera because I want something that will show off my vocals. Does anybody know of any other good artists or songs that I could possibly use?