A chainsaw. Batman frowned, but wasn't too concerned. In his experience, it was the weapon of an amateur, someone more fascinated by blood than serious about fighting. However, it was also often the weapon of a lunatic, and insanity always made someone unpredictable and more dangerous because of it.
He secured the batline and tested it with the ease of long habit, paying little attention to something he'd done so many times, then launched himself straight at the maniac with the chainsaw, feet pointed to strike his chest with enough force to knock him over and bruise ribs.