There is only one reliably open buffet in the Magic Kingdom, The Crystal Palace. However there are also two other buffets that can be open under certain (rare) circumstances.
The Diamond Horseshoe is open seasonally, so it may not be open when you visit. In the daytime it is a quick-service restaurant, however at dinner time it is a buffet. As a buffet it serves food similar to the Liberty Tree Tavern dinner.
Similarly, the Tomorrowland Terrace Cafe can also be difficult to predict. Unlike the Diamond Horseshoe it is always a buffet (when it is open). It serves what they refer to as “American food with a twist.”