I pick ARIEL because all she'd have to do to beat up any of the other Disney Princesses is grab them by their hair and drag them underwater til they drown and DIE. Bwahahahahahaha!!
Also, she's got a beautiful voice and red hair and she's strong and independent and doesn't let a little thing like Giganto-Ursula get in the way of true love. Plus she used a fork to comb her hair. And she talks to fish. And she has her own sequel AND cartoon series and because....uh...she's the favorite of six kids.