Fellow burger lover here and also a big fan of the burger at Boathouse and agree that eating it at the Dockside bar is preferable.

The wagyu burger on the lunch menu only at Hollywood Brown Derby in the Studios is another favorite. Unfortunately currently closed, but I also love the burger at Crew's Cup Lounge (adjacent to Yachtsman Steakhouse). The burger at Ale & Compass (also in the Yacht Club resort) is a good one as well. The bison burger at Geyser Point is next on my list to try. I've also heard good things recently about the burgers at Homecomin', City Works (both Disney Springs), and Rose & Crown (EPCOT).