I've had Burgers at Espn club to the Sci-fi to every Park. Seems that the Burgers at Pecos Bills always taste the best. maybe its beacause of the fix-ins Bar. Just My opinion, what are other peoples?
This may be a bit out of the way, but the best burger I've eaten at WDW is at The Mara (counter service restaurant) at Animal Kingdom Lodge. Their double bacon cheeseburger is great. And they have so many other choices for meals - and a cold food case too!!