Kelly,
There are a lot of great choices, but since you referred to her as a princess, there really are only two choices.
Cinderella's Royal Table in the Magic Kingdom, and
Restaurant Akershus at Epcot.
The first one is the most popular dining experience at Walt Disney World, and is typically a hard reservation to get. But now that they have extended the character experience to not just breakfast and lunch, but to dinner as well, it is easier to get a table there now. They have also improved the experience, by including four other princesses at the meals now.
Akershus is a fantastic experience as well, with several princesses holding court at any of the meals. The setting is equally regal, and the food is good too.
Either of these would do fine for you, but know that the Cinderella option is more expensive, and requires two table service credits, where Akershus only needs one. Since the recent changes to the experience at the Royal Table, I would recommend you try to get that one first, if it suits your budget. But know that either will be a wonderful experience.
Have a magical (and delicious) vacation!