A poleaxe.
I was fortunate enough to be invited to the Royal Armouries in Leeds a few years ago and two guys came into the room in full armour and proceeded to try to knock seven bells out of each other with poleaxes. This was an excellent display and they were extremely proficient. Afterwards they were both fine of course (and really knackered

), but afterwards one of them told us tales of bodies from mediaeval battles being excavated by archaeologists many years later and the poleaxe injuries were horrific. Like cleaved in two from the top of the head down to the sternum!

This is why some helmets of that period go to a point at the top in order to deflect the poleaxe. The poleaxe is not limited to axe work of course, it also has hammer face on the opposite side of the axe blade, and at the other end it goes down to a sharp blade for stabbing. A very nice piece of kit in the right hands.
I said I would have liked to learn to use one but he just smiled and said they weren't offering courses in using the poleaxe.
Shame.
Edited to add: The police would probably confiscate it if they found you carrying one.
