Here's my top five:
: nothing like standing about 60 ft. below an arriving 747! (or a couple-hundred feet behind a departing 747, clinging onto the fence as over 200,000 lb. of thrust get blasted out of the engines)
: massive airport, great variety, lots of 747's, great weather.
: see LAX
, minus the good weather, plus even more 747's.
: great variety from airlines all across the world (especially Europe, some of the airlines quite exotic such as Aerosvit).
: see LHR
, minus some 747's, plus lots of RJ
's and a near seamless stream of arrivals and departures.
doesn't make my top five, but is a close #6. Its extremely busy like ORD
and has lots of (Delta) widebodies, especially (Delta) 767's, but lacks variety as its overwhelmingly Delta, quite a bit of AirTran, and then everyone else (all the legacies plus a few int'l carriers).