Three questions decide the material: where the part works, how much load it carries, and what environment it sits in.
In damp or chemical environments stainless is unavoidable; it costs more up front but lowers replacement frequency. For load-bearing parts in dry conditions, plain engineering steel is both cheaper and sufficient.
Aluminium gives light weight and fast machining, but wears where threads are cut and assemblies are opened often. Brass and bronze come forward in sliding bearings and connections that need sealing.
Our approach: you do not have to specify the material. Tell us where the part works and we will propose and source the right one.