Metro series (very good, contains creaures/mutants but also a lot of enemies are human)
STALKER series (very good, contains creaures/mutants but also a lot of enemies are human), some mods recommended because the base games are a bit "unfinished" for lack of a better word, but the modding scene is great and always kind of saves the games
Deus Ex series (I've played 1, Human Revolution + Mankind Divided, all very good. Part 1 in particular was legendary back in the day, unfortunately it looks like absolute garbage these days (it was already critizized for its dated look back when it released), but you might find some mods beefing it up a bit)
Wolfenstein series (the newer titles are very good in general, except Youngblood)
Dishonored series (very good)
F.E.A.R. series (great horror/scifi themed FPS)
DUSK, Ion Fury, ... (and in general lots of other boomer shooters, lots of them are great, these are just 2 examples which I fondly remember. Lots of high quality indie games in this category)
Cyberpunk 2077 (as an FPS-RPG hybrid, very good since version 2.x, especially with Phantom Liberty)
Sniper Elite series (I've played 2-4, they were very decent)
Far Cry series (I've played 1, 3-5 + New Dawn + Primal, they were very decent. New Dawn was more like DLC-sized)
The newer Terminator FPS games are supposedly good, haven't played them yet
The new Robocop FPS is supposedly good, haven't played it yet

These are more controversial but also can be a fun option:

Deathloop (interesting conceptually but also not amazing. Only get it at heavy discounts)
RAGE 1+2 (don't remember much from 1 anymore, but I finished it so it was at least okay. 2 was short and unimpressive but still works as well, it's nothing impressive but if you just want a sci-fi singleplayer shooter they're alright I guess, and they're probably very cheap these days. Only get them at heavy discounts)
The Division 1/2 also work decently in single-player, although some late-endgame stuff like Raids requires multiplayer. Might not count as FPS since it's 3rd person by default, but it plays very close to FPS and IIRC you can change camera view to 1st person and when you aim from cover (which is most of the time) it feels like FPS regardless. I played both for a bit and it was better than expected but it's also bland and gets boring after a while. Still a somewhat decent "realism"-based shooter. Only get them at heavy discounts.
Theoretically Call of Duty but I'm still not going to recommend them because of unethical developer or publisher, low value in general, inflated prices in general, short lengths in general, and so on.