The BFG of course!
Hmmm.....
Toadstool's "Frying Pan" in Super Mario RPG was the first thing to come to mind. Given the best setup, you never use her for offense, but it was her best weapon. A frying pan.
The other thing that came to mind was Frank in Shadow Hearts: From the New World. He literally stuck a hilt on whatever object he could find, be it a stop sign or a freaking cactus. Yeah, that's how you upgraded his weapons.