I know it is not a Restaurant
but the Ben and Jerry's in Terminal C of IAD was my worst food experience. Service is awful and the place is dirty. Usually the same person who handles the ice cream handles the money. I got a cone when I first started travelling there was debris on ice cream. I through it away and never went back.
A few weeks later I overheard 2 airport employees when riding the moon buggy. One of the guys was obviously new to the job and the other more experienced. The jist of the conversation was the one guy telling the other that all IAD employees knew to never eat the ice cream there. That what goes on behind the scenes was nothing compared to what the counter looked like.
I haven't been to Dulles since August and when I last saw it that area was under construction. Hopefully whatever comes back is cleaner.