What is the best melee armor in runescape?

Best for what? It really depends on a variety of factors; like how much money you want to spend, and your preferred combat style. Personally, most of the time I try to maximize prayer bonus, to make prayer last longer - and then use protection prayers. This type of armour is fairly cheap. You can also choose armour that maximizes melee damage, ranged damage or magic damage you cause; or concentrate more on armour that protects you better, so you can use less food while attacking NPCs. There is usually some trade-off, meaning that armour that is best for one purpose is not necessarily best for another. For example, if I maximize prayer bonus as mentioned above, I also cause less damage per second, since I have less bonus for melee damage.