Worst defect? Difficult to choose, but I recently refused to send a student solo because the stall warner was u/s. I do worry about the casual attitude of some schools.
However, the "defect" that really annoys me is missing paperwork. I now refuse point blank to fly any aircraft with any of the paperwork missing. I'm fed up of hearing "the engineer says its ok and has done the 50h check but he was too busy to do the paperwork before the aircraft came back".