What is a cyberman's weakness?

Answer 1

Their emotional inhabbiter located in their chest. It makes them remember who they are and they have a mental breakdown and die.

In older eps they are shown to be allergic to gold. A gold-tipped arrow is shown to kill one instantly.

Answer 2

As far as I know, no specific weakness is mentioned in the television series. It seems that the cybermen become harder to fight over time, since the tend to upgrade themselves and overcome any weakness they may have had.