Hi Tiffany,
Of course the term "best" varies depending on who you're talking to. No steakhouse would be the best restaurant to a vegetarian and a sushi spot could never be the best for someone who hates fish.
That being said, the very best restaurant at Walt Disney World is
Victoria & Albert's in the Grand Floridian. The mulit-star restaurant is known for being one of the finest eateries not only at WDW, but also in Florida and the entire country. I've had the great, great privilege of dining there six times. I've never had a single disappointing bite. Each morsel is crafted with care from the finest ingredients anywhere: caviar, Dover sole, Waygu beef, and so on. It's quite pricey but, in my opinion, oh so worth it.
The next step step down are Disney's
signature restaurants (click on the link for a complete list). These are the "regular" gourmet venues, with wonderful service and incredibly delicious foods. My favorites on the signature dining list are Artist Point and Jiko, but I've had good experiences at all of them.
For more detail about dining at WDW you may want to check out the independent website DisneyFoodBlog.com which has reviews of all the restaurants as well as many photos of the fabulous food.
Happy eating!