- The classic perk system: You have a four-perk limit, which can be expanded by certain free perks on different maps (Appears in WaW, BO1, 2, 3, and WW2)
- IW zombies perk system: You have a 5 perk limit, are able refund any perk you have to get points back and replace that spot with a different perk, and every map in the game has a a unqiue side EE that, if you complete, gives you all perks on the map for free (Only appeared in IW zombies)
- BO4 perk system: You can choose your four perks before the game. The four classic perks still exist, but not as perks, but through different things (everyone starts with 5 hit down, you need to drink your 4 main perks to get faster reload, etc), and you can only get 6 bonus perks in a game through EE or Perkholic. (Only appears in BO4)
- Modern perk system: You can buy all perks on the map, but the price increases by $500 every time you buy one. You can add upgrades to your perks via augments (In BO6 and 7) and the Aether crystal tier system (CW), which will make your perks have unique abilities (including old perks powers like electric cherry) or just do what they do, but faster or stronger (Appears on CW, BO6, and 7)
- Vanguard perk system: In Call of Duty: Vanguard, the Perks are now known as Demonic Fountains, featuring new Perks which are similar to older, previously featured Perk-a-Colas. Perk Fountains feature 4 Tier Upgrades with the base Tier being free and every subsequent Tier having an increased cost: 2500 Essences) for Tier 2, 5000 for Tier 3, and 7500 for Tier 4. When downed, the player will keep the Perk but will lose one Tier. (Yes I copied from the COD wiki because I don't like Vanguard)
- MWZ perk system: Think Modern perk system, but with out augments and CW tier system
My ranking would be:
IW>Classic>BO4>Modern>MWZ>Vanguard