Chainsaw. No if's and's or but's.
I mean seriously! Think about it, if you were about to jump someone and you were thinkin' 'Oh hey, this will be easy.' And then suddenly *REEEEENNNANNNANNNANNNANNAAAHHNN!!!1!ONE* 'EEEEHHHEEEEHHEEEEEHEEE!' (because I would have a crazy laugh when I start up a chainsaw in a dark ally) I am pretty sure that the first thing that goes through your mind is 'Someone just crapped in my pants! Oh wait, that was me.'
So yeah, I'd go for the 'let's scare the crap out of our enemies before they have the chance to even touch me and see how weak I really am' choice.
Flamethrower or tazers would be second choice for me, but those weren't options.