The correct answer of course is the one down the road.
But, looking at busy airports, the top ten are :
ATLANTA (ATL)
CHICAGO (ORD)
DALLAS/FT WORTH AIRPORT (DFW)
LOS ANGELES (LAX)
LAS VEGAS (LAS)
HOUSTON (IAH)
DENVER (DEN)
PARIS (CDG)
PHOENIX (PHX)
PHILADELPHIA (PHL)
http://www.aci.aero/cda/aci/display/...=1-5-54-57_9_2__
The best of busy airports would have to be Phoenix. Not a bad airport at all in fact, though I am sure the weather helps. DEN is a bit high when running for a tight connection. But given the massive number of flights these US airports handle they do a very good job indeed.