Our favorites have been the "Kobe" burger at Yak & Yeti and the burger at Hollywood Brown Derby. Seems odd that one of the best burgers on property would be at a predominantly Chinese restaurant, but we had read about it and gave it a shot...and have returned a few times for the burger. We had the slider version of HBD's burger outside in the Lounge, and it was pretty incredible...I'm assuming (with that assumption strengthened by seeing other reviews) that the regular burger -- available only at lunch, I think -- is equally delightful. To be honest, I would not typically order a burger at a place like HBD that has other "nicer" options, but since it was one of the few options on the basic Lounge menu, we tried it and absolutely loved it. I would seek it out now.
Without the benefit of actually having eaten at the ESPN Club on the Boardwalk, I would argue that they have the best selection of unique burgers on a menu. Would love to hear if anyone has tried any of them and if they are as good as they are unique.