Without a doubt it would be Trails End over at Fort Wilderness. That's a superb breakfast with the biggest selection of food I have found at a breakfast buffet! The feel of the place is great too.
If it is a MK day. Kona Cafe and Grand Floridian Cafe are both good and on the monorail. Epcot or Studio day, Capt Grille is good. Rainforest Cafe at MK breakfast is huge but the quality is lacking IMO