SAE 316L stainless steel

It's very easy to work with and very strong without being too heavy and is also relatively cheap.

There are a few different alloys of stainless steel with variants in hardness and corrosion resistance but in watchmaking it's 316L that's most commonly used.

Rolex uses 904L which is more corrosion resistant and shinier.

Stainless steel is basically the benchmark against which all other materials are measured since it is by far the most abundantly used.