There's also Batgirl, Black Widow, Black Canary, Storm, Rogue, Mystique... You could probably do villianesses as well, so why not Harley Quinn? Perfect excuse to go on a rampage of sexy silliness.
I'd go with Wonder Woman, Supergirl, or the Invisible Woman (Susan Storm) just cuz all of them are recognizable and iconic.
if you have seen the show arrow (awesome show by the way) you should dress as the canary. her character is different there will probably be like 25 wonder women, bat girls, or super girls but just have fun with it.
