Lightweight | Lightweight Pro
Ability: Allows you to move faster, but how fast depends on the equipped weapons mobility. This should be used with a "run and gun" class due to the added speed.
Pro Ability: No falling damage.
Scavenger | Scavenger Pro
Ability: Replenishes your ammunition and lethal grenades when you walk over blue scavenger bags that are dropped by dead players. This won't resupply launchers or equipment. Scavenger is very useful with weapons that run out of ammo quickly.
Pro Ability: Doubles your starting ammo with extra magazines. This also replenishes your tactical grenades.
Ghost | Ghost Pro
Ability: Ghost works pretty much like Cold Blooded from Modern Warfare 2. You are undetectable by the Spy Plane killstreak when you have this equipped. You show up on the radar if you fire a weapon that doesn't have a silencer.
Pro Ability: Undetectable by aircraft, infrared and sentries. Enemies won't see your name or a red crosshair when targeting you.
Flak Jacket | Flak Jacket Pro
Ability: Flak Jacket greatly reduces explosive damage and allows you to survive any indirect explosive attack. Explosive crossbow bolts are lethal if they get stuck on you and explode.
Pro Ability: Deflects fire damage and allows you to reset the fuse of a thrown grenade when you pick it up
Hardline | Hardline Pro
Ability: Reduces the amount of kills required for any killstreak by one. For example, the Care Package normally requires 5 kills, but if you have Hardline equipped you only need 4 kills.
Pro Ability: Allows you to randomly choose another reward from a Care Package. When you open a Care Package, you can either take the reward or try again for something better.
For example, when selecting a Care Package and you get the Spy Plane killstreak reward, you get the following option: "Hold X for Spy Plane OR Double Tap to Try Again" (Xbox 360)
So which one do you guys prefer for perk slot 1? I myself mainly use. Ghost or Hardline. these 2 seem way overpowered compared to the rest IMO.