breast plate will be good if you haven't yet made your hammerdin... or put any points into strength... such as if you got a grush, and then just saved your stats/skills to use when you are ready.
archon is good because with a strength:defense ratio, it offers the best combination + its a light plate
my personal favorite is the mage plate (exceptional version) 55 str req. and can still get 1k+ defense on an enigma that way you shouldn't need to rely on charms to where your equipment