Were you thinking in the parks or a nicer signature restaurant at one of the resorts?
If you want to dine in the parks I'd choose somewhere that had fun drinks since its your 21st. Maybe Morocco where there's a belly dancer or Germany with big steins of beer.
Any of the signature restaurants would be very good if you want something like seafood or a steakhouse. It depends on pricing and ambience that you want. A quiet dinner with a boyfriend or husband, your whole family, or a group of your friends.