What is the perfect dress to wear to a formal event like this?

I am going to be attending a formal banquet for Academic Decathlon and I have three dresses : a black one with a blue busted top (one strap going across my shoulder), a purple one that is flowy and has a black bow on the bust, or a clic LBD little black dress (casual enough to have pockets) but still very nice.. i dont really know which one to choose..any suggestions would be appreciative:)