Best Pack A Punch system?

• WAW-BO1: Simple.
• BO2: Attachments.
• BO3: Alternate Ammo Types.
• IW: Double Pack.
• WW2: Buy Ammo.
• CW-BO6: 3 Tier Pack.

I always liked the IW system. Doing a quest to unlock even better guns just felt right to me.