LAX and Ohare can be pretty trying, but of the new ones I think Denver is a hassle, if for no other reason than the runways fell like they're in the next state over from the terminal. I think you spend more time taxing in Denver than at most other airports. Now Detroit used to be a bitch if you were flying on anything other than Northwest (now Delta) but the new terminal, where the other airlines have gates, is very well done.
Now the best airport that I've flown in and out of in terms of being efficient has to be Singapore. Now most of the big airports in Asia are very nice, but Singapore is just amazing how quickly they get you from the plane, through immigration and customs and then out to your cab. The last time I flew there I timed it. From the moment I stepped out of the door of the plane until I was getting into my cab was 20 minutes.

OCU