We like both Sci-Fi and Beaches and Cream for burgers and onion rings. Pick the most convenient one. Remember that B & C does not take ADRs if that makes a difference.
I personally have not been, but a great friend of ours recommends The Wave at the Contemporary Resort for "one of the best burgers ever." Just a thought.
I know nothing about burgers, but the onion rings at Sci-Fi Dine-In are fantastic!!! You can see a picture of them and read more in my review (click the second link in my signature).