Which is strongest material- Mild steel or stainless steel?

Mild steel. The lattice of atoms in the Fe is strongest when the fewest "impurities" (such as carbon) shear that lattice structure.Stainless steel is not made for it's strength but for it's non-corrosiveness and ability to hold an edge. Stainless steel is iron with chromium and nickel and carbon in it and all of these atoms disrupt the tensile strength of the iron lattice structure.

Mild steel is just iron with low amounts of carbon in it, but wears down and oxidizes more (rust) than stainless, but is useful where these qualities aren't needed.Rebar used to strengthen concrete, uses mild steel for the bar because it is stronger tensilely and much cheaper and is protected from rust by the encasement of the concrete.