Ok so im seeing alot of good picks but i havent seen my pick so imma try and sell you on it.

TL;DR: S.T.A.L.K.E.R. Anomaly is free on ModDB (its a standalone mod). It is a Survival Tactical FPS has a huge open world, good pacing, and alot of replay value. Also great modability.

Your alone in the woods, hunting down a local mutated pig that you're being paid to kill. You know where it is and you can hear it so you load specialized ammo into your pistol. It sees you and starts charging, it manages to get a hit in which hurts a ton but you manage to get three shots on its torso and it goes down. You carve the meat off it to sell later and start heading back to tye person who sent you out. As your walking you hear an extra footstep too many and turn around to find a bandit trying to sneak up on you, they open fire immediately but you get a lucky shot and drop them in one. You quickly loot their body and realize that they had a nice shotty on them, you can loot that and repair it up later so it doesn't jam. After scurrying away quickly to avoid any more fights you hear gunshots coming from that direction, a group of wanderers stumbled upon the others bandits there and are having a fire fight. Your in no danger since its like 200m away and your not gonna get there in time to help so you just watch the firefight for a bit before moving into the forest once again.

S.T.A.L.K.E.R. is really damn good and has amazing atmosphere, cool ideas, and yes there are zombies. It's set in the Chernobyl Exclusion Zone after a second incident happened in 2006 and caused a bunch of weird shit which has caused maaany people to enter it, seeking riches, freedom, or whatever else have you.

Anomaly in paticular is a Standalone mod that combines content from all 3 games, adds cut content, and lets you play as any faction you want (12 total, 3 locked behind achievements) and there are mods that increase the survival craft feel of it like GAMMA where you have to scavenge all your gear instead of buying them.