I want to say Maldonado, but he’s been doing a comparable job to Barry this season and seems to have cut out a lot of the mistakes that he was doing early on.
D’Ambrosio is in what I think is the worst team at the moment (yes, worse than HRT), so it’s difficult to judge him, but as they say no news is good news, and we haven’t seen much of him this year.
Massa would probably be my choice. As an experienced driver he has no excuse for the errors he’s made this year, many times he’s finished well below what the car was capable of and so often he falls back due to bad luck, but then makes no headway. He might not be the least skilled driver on the grid but I think his performance is lowest relative to where he should be.