What is your preference, experience and why with different materials.
I started with aluminum Lee molds because of the price. So far I haven’t seen any problems with them. I have a Pedersoli mold in steel that casts great round balls, but also a Lyman in steel that casts crap, mainly because the mold has tooling marks. I haven’t yet owned a brass mould, but MP molds in brass, are looking interesting.