CVG doesn't have a LCC unless you consider ComAir one.
Let's redefine the question. If we are looking for the largest LCC operation in one airport LAS
comes to mind thanks to AWA and SWA.
If we are looking for the largest airport that is served by LCC than ATL
it is since it is served by ATA. If the largest US airport happens to be ORD
, then that is the answer since it is served by Sprit and AWA.