It's a toolbox, man. We need parameters. Many folks here have been playing for quite awhile, so we have a bit of weapon stuff under our belts.
That being said... I'd be cliché and say "My Mind!" but it's too obvious, and no one would ever stand a chance!!!

(What're we gonna do tonight, Brain?)
I would say that the most lethal weapon, barring any firearm, would be a razor sharp knife or dagger, at least 4" long, wielded by someone skilled or not, but so committed to killing you that they temporarily lose any semblance of self-preservation instinct. THAT adds up to a bad day for everyone involved.
I picked this because of it's a prevalent weapon, easy to conceal, pretty difficult to disarm, and you usually don't know that you're being assaulted with it until it's too late to avoid getting at least hurt.