Kedrosky votes for London Heathrow (LHR), but I'm here to tell you that Frankfurt (FRA) is the worst airport in the world.
Here's why Frankfurt is 2X the crappiness of Heathrow.
The one good thing about Frankfurt - the beer and pretzels in the Lufthansa first class lounge.
Heathrow has one special charm to offset its many problems: imagine you have arrived on a red-eye from the west coast. You're getting in at 6am. You get off the plane and onto a bus. Normally I hate buses after the plane - i.e. Dulles. The bus drives thru the bowels of the airport (on the wrong side of the road). You are barely awake, but the sensation of driving thru a life-size LEGO airport, with all the little workers in colorful jumpers, driving LEGO like vehicles around, is something that amuses me every time....
I know, I'm weird, but there you are.