Realistically I'd probably go with an axe. It works as both a cutting and a bludgeoning weapon, and it's heavy and powerful without being too cumbersome. Plus you could use to chop firewood (it's the apocalypse, you'll need to keep warm)
Unrealistically, probably a lightsaber. You can swing it anywhere, and chances are if you don't kill it you'll cripple it.
Unrealistically, probably a lightsaber. You can swing it anywhere, and chances are if you don't kill it you'll cripple it.