Never worked DXB, LHR, or ATL... But I did work at two other top 10 airports...
As I understand it, DXB works more heavy's percentage-wise then anywhere else in the world... to usually one runway.
No clue about LHR... Never been there, don't know anyone there... but they do have parallels that are far enough apart to run simo's... something DXB cannot do.
ATL has 5 runways, of which 3 are used for arrivals at any one time... Including simo's during IFR. Not knocking my Brothers there, 'cause they do a great job.
All I am saying is total number of movements does not always equal who (ATCO'S) is the busiest. It also depends on how many controllers are working those planes.
IMHO, if you are certified at any busy international airport, you are pretty equal in my book, and have my respect...