What metal could defeat some versions of Cybermen?

Answer 1

The original Cybermen (Classic Who) could be defeated with gold.

The NuWho Cybermen are not weakened or destroyed by gold as they were able to finally upgrade past that flaw, however, they could be killed or "deleted" by using their own weapons against them or if they were shot by Daleks.

Here is a list of some other things that can hurt or kill them now :

gravity and solvents based weaponry, radiation, and EMP based weaponry

Answer 2

Gold is often depicted as the weakness of some versions of Cybermen, particularly those from the Doctor Who series. Gold can short circuit their systems and weaken their cybernetic components.