I am overthinking, but are we talking fictional or real world ? Melee or range ? Do firearms counts ? If is only deadly on the hands of a skilled user, does it counts ?
since someone already said buster sword i will say two more. the senpumaru from onimusha 2. lightsaber
To the untrained eye,.....they may look like a walking cane / stick.....but watch closely in this video on the Irish Walking Stick.... It so happens,....that after seeing the demo on the Irish Stick....I got one. I FREAKIN' LOVE THAT THING!!!! A walking stick to some.... A WEAPON IN MY HANDS!!!
I bought this a couple of months ago: Looks like an assault rifle but in reality it's just an overgrown 9mm.
One of my personal favorites is the kanabo. You may or may not recognize them as the big metal clubs (often spiky) that are often used by Japanese oni, or demons. Perhaps not the most efficient weapons, but I find that a lot of times efficiency is inversely proportional to badassery.