United. They merged with Continental Airlines and totally messed up my reservation, changing our connecting flight out of Chicago to MCO, so we would not make our connection. They removed our "Economy Plus" seats, that I paid extra for and changed our flight times significantly.
Thank goodness I booked with Travelocity, they were able to help us change our flights, refund our "economy plus " fees and received a travel voucher for United for a future flight. All in all it took 2days of phone calls to change our flights and seats and 4 weeks to refund our fees and get the voucher. It was not a great experience.
Since then, they have messed with our flight times, changed planes (then moved our seats to the back of the plane) 3times in the last week!