Minecraft wiki entry for Player_versus_Player, associated text: Gold armor Many people don't use this armor, and it shields for 44% of damage, if fully worn. A diamond sword critical hit will do 6 hearts on it.
Chain armor This is the least used armor because it is so hard to get. Protecting for 48% of all damage, it is about as effective as gold armor.
Iron armor The third best armor and one of the most effective sets of armor (due to iron being easy to find) iron armor seems fairly weak to be the third-best armor. It protects for 60% of damage taken.