Chicagos O'Hare Airport (ORD) was the busiest hub, however many years ago. It still seems to me that it is the busiest. The Chicago Approach/Dep. controllers also must fit in Chicago Midway's (MDW) traffic, which is very close by.
But in Atlanta (ATL), all of the runways are parallel to each other 08L,R, 09L,R-but this creates problems with the entire operation depending upon "hold short" clearances
. ASA (no longer at DFW) and AirTran also have their hubs there.