Hardened | Hardened Pro
Ability: Increases your weapons bullet penetration, but it doesn't increase bullet damage. This is pretty much like Deep Impact from Call of Duty 4 and FMJ weapon attachment from Modern Warfare 2.
Pro Ability: Your weapons do extra damage to aircraft and turrets. This also reduces flinch when you are shot by someone.
Scout | Scout Pro
Ability: Enables you to hold your breath longer when steadying a weapons scope. Scout offers a significant advantage to sniper class weapons.
Pro Ability: Allows you to switch your weapon faster.
Steady Aim | Steady Aim Pro
Ability: Steady Aim increases a weapons hip fire accuracy and also reduces shotgun spread. This should be used with the Dual Wield attachment, since Dual Wield significantly reduces a weapons hip fire accuracy.
Pro Ability: When equipped, this enables you to aim faster after sprinting and recover faster from a knife lunge.
Sleight of Hand | Sleight of Hand Pro
Ability: Gives a faster reload time by reducing your weapons reload time in half. This is extremely useful with weapons that have long reload times, like shotguns and light machine guns.
Pro Ability: Allows you to aim down the sight ~50% faster than normal. This doesn't work with weapons that have scopes.
Warlord | Warlord Pro
Ability: This is the equivalent of Bling from Modern Warfare 2. Warlord allows you to put 2 attachments to your primary weapon. Stealth classes should use this since they can equip a silencer to remaining undetected and still reap the benefits of having a second attachment.
Pro Ability: You start with one extra lethal and tactical grenade, except smoke (WILLY PETE).
Sooo... which perks do you all prefer for slot 2?
for me this on depends. SMG classes i use steady aim mostly. For all other classes besides sniper i use either Sleight of hand or Warlord. For snipers i use the obvious Scout.