Modern BattleMech armor is comprised of multiple layers designed to protect the machine and its pilot against energy and projectile weapons respectively. When damaged, the armor shears off and must be replaced. Ferro-fibrous armor is a variant armor that weights less, but takes up critical spaces.

The strength of the weapons against the strength of the armor is always important. Take into consideration the 'mechs you go up against. That'll help you what you need, armor-wise.