I've always loved the Bison burger at Geyser Point, but during our most recent visit last week, I had several burgers including both the Bison burger at Geyser Point, the Stack burger at Steakhouse 71. I was actually somewhat disappointed in the Bison burger during this trip as it simply didn't taste nearly as good as the other times I've eaten there and seemed overcooked. In comparison, the Stack burger was nothing short of amazing for a burger on Disney property. It was cooked perfectly and the combination of the house made pickles and lemon aioli really added to the depth of flavor for this burger. Not to mention, the parmesan fries made a great companion side item.
Also FWIW, the Floridian Eggs Benedict and Walt's Prime Rib Hash were equally amazing at Steakhouse 71 during breakfast. I was very impressed with Steakhouse 71 and the wait staff, especially Grant, provided fast and friendly service.
Also a good friend of mine and a local tells me that the Gibson burger at the Boathouse is by far the best burger on property, although they've recently taken it off the menu.