What armour is best for a member on runescape that's lvl 40?

Well it depends what your defense level is, and what type of monster you will be fighting. So for example if you were to train on Varrock guards the best armor to wear would be a plate body(no chain mail), plate legs, full helm and boots of that armor type(depending on what armor set you are wearing) because sWords use stabbing attacks, and plate armor has very good defense against stabbing attacks. The ideal weapon would be a mace, because varrock guards wear chain mail which is weak against maces as it has low crush defense.

Please apply this to other monsters, as the varrock guard was just an example.