Classic Marathon 2

Many people, myself included, feel that Marathon 2 has the best campaign out of the Marathon trilogy; there's an excellent story, varied level objectives, fun combat, and an interesting setting (you're exploring the ruins of an alien civilization).

It adds a layer of depth and tension to shooting not present in games like Doom, and it's a good mechanic at mitigating Marathon 2's stagnant enemy variety relative to the first Marathon.

Part of the game's great visuals that I'll never get over are the skyboxes.

Marathon 2 only improves on the combat by giving more of a purpose to the Zeus energy pistol against armored and mechanical enemies and adding one of the best shotguns in video game history and letting you dual-wield it.

Multiplayer functionality is also included, offering both cooperative and competitive modes that echo the early days of networked FPS gaming, even if the active player base today is relatively modest.

I'm having a great experience with the 2 player coop despite a tiny bit of jank (but what can you expect from a game that's almost 30yo).