All three kinda abused the position imo, although Graham only did it under Regina's control. When she wasn't commanding him he seemed to take the job super seriously, and apart from the car chase with Owen's father (which again, wasn't his call), everything he did was legal and seemed to follow procedure.

Emma and David didn't do anything terrible as sheriff that I can remember, but they didn't seem to take the job seriously enough, and they sometimes acted immorally in their position 'just because.' For example, in season 4 Emma held Will in a cell indefinitely for, as she openly admitted, 'crashing her date.' It was played off as humor but honestly that's kinda messed up.