Zombie 2100: The original
Zombie 2100 is a great introduction to the series that teaches you to the casual retro survival game.
Zombie 2101: Daily challenge
Zombie 2101 updates daily with a different setup and adds a leaderboard.
Zombie 2102: The opponent
Zombie 2102 adds an AI opponent that can cooperate to kill zombies, or murder you. It also adds a bleederboard for those who did not survive.
Zombie 2103: Fire! Fire!
Zombie 2103 adds the ability to ignite zombies at your location with gas.