I'd have to say one of those ASP expandable batons. When you carry a knife, while it is more effective, you'll usually hesitate to use it since it's such a messy weapon. With a baton it doesn't cause so much permanent injury, and there's less of a chance of death, so you won't hesitate as much. Even so, I carry a Buck folding knife that has the quick open stud on the blade. It's about the closest legal thing to a switchblade. I hope that I wouldn't hesitate to use it, but you never know.