I'd say uranium would be pretty cool. Depleted uranium is abundant, it's pretty much mixed in with lead wherever lead deposits are. Especially, if it was used for arrowheads or something. It's something that's easily attainable for middle ages as uranium was used to color glass, and with it's penetrative properties, and high density could make for nasty arrows. Unfortunately it's radioactivity is exaggerated, and it really isn't much worse for you than lead is if it's ingested.
As far as other metals, titanium has poor properties as far as blades go. Could make for some good armor though if you could figure out how to process it due to it's strength and light weight. Also, because it's neutral in the body, its not what you'd want to use for arrows, because if the wound is clean, the fragments of titanium won't cause a lethal infection.
If you could ever isolate polonium, THAT would be a nasty weapon edge, especially if used in arrowheads designed to fragment. It's a strong enough alpha particle emitter that it would kill a person within a few days if every little speck wasn't removed from the body. This would be almost impossible with middle ages tech. It'd also be a nasty poison.
Radium could be cool if it was laced into a sword. The sword would glow in the dark for one(a cool shade of yellowish green), maybe the radiation emmited from the blade because of how strong of an emitter radium is, could be deadly to certain kinds of monsters(the medieval people thinking the "light" was what killed them). Plus the weapon would be safe to use as long as it was kept sheathed until you wanted to use it(it's a strong alpha emitter, which should be safe assuming you don't lick the sword).