There are several generations of armour there.
If you like assault mechs, hardened is great, but it makes you slower, piloting +1. In exchange, double effective armour, but 1 point BBs deal double damage (1 point marks off 1 bubble, which normally 2 points would).
Reactive is great against ballistics, but does nothing against energy.
Reflective is great against lasers, but nothing against ballistics, and takes more damage under certain circumstances.
Ballistic Reinforced reduces damage, including negating BB weapons.
Honestly, if you can, go with Ballistic Reinforced. But if you just need armour, have good pilots, and are ok with slow, hardened. If you border with Davion space, go with Reactive.