I would nominate DL with UA being a close runner up.

DL has surly phone reps that have little knowledge. On a recent trip I had problems with a rep in Mexico City not knowing the difference between Manchester, NH and Manchester, UK

The number of seats available to destinations that you actually want to visit can only be described as tiny. The SkyTeam alliance can best be described as a joke with AF, AL and KL seeming to want out so badly that they will tick off DL customers to no end by having virtually no seats available in premium classes, even a year out.

UA on the other hand has seemed to corner the market on trying to foist off bad destinations to people that have no choice because of expiring miles or schedules that can not be changed.

Any other nominations?