Lambert-St Louis.
I have never had an enjoyable flight there.
Don't know if it is the lack of adequate air conditioning in the summer, proper snow removal in the winter, a decent engineered layout, or the worst, most obnoxious security guards ever, but I really can't stand STL.
Plus, I have NEVER had an on-time flight through there. The closest was only 45 minutes late on take-off, and that was couple of weeks after September 11th when all the airlines were running on-time.