My two least favorite for arriving or departing are LAS and MCO, mostly because they're full of amateur travellers. Security lines are abysmal at both, too many people with too few manners and not enough common sense. I get the feeling that the respective states/cities don't feel the need to improve the airports that much because people are going to fly through there anyway; there's no competition and huge demand, therefore no need to improve. Baggage claim at LAS is painful and I won't let anyone I travel with check bags. At MCO by the 50-59 gates they're doing some remodeling and have taken down the ceiling panels, exposing the pipes and wiring, all caked in dark grey fuzz stuff, probably air pollution gunk, and it just looks filthy.

Runners-up: IAD (it's always a zoo, morning noon and night, and their tendancy to deice at the gate instead of in the deicing pad (I'm not even sure they have a deicing pad) leads to lots of really ugly delays during winter weather... stuck on the ground waiting for a gate... and LGA is just a dump.

Favorites:

DCA-- clean, pretty, well-organized, they could use some more concessions past security but all in all, things run pretty smoothly at DCA. Plus it's home so I am biased.

MEM-- I was kinda surprised by how much I liked MEM. The newer terminal is really nice-looking and there's a good traffic flow.