MK--The Plaza, with Cosmic Ray and Pecos Bill a close second
MGM--SciFi is great. The burgers at ABC Commissary are TOXIC.. BAD BAD BAD
AK--no burgers at AK. Great counter service at Flame Tree and Tusker, but not
a place for good burgers.
Epcot--the grilled steakburger at LeCellier is probably the best burger in Disney.
As far as Electric Umbrella, see note above on ABC Commissary. IMO,
the American pavilion at Epcot is embarrassing to us Americans. All the
great international food in Epcot, and we have precooked hamburgers
under heat lamps.
Honorable Mention: Beaches and Cream, but not worth a special trip.