System Shock 2. It's hard to get through reputable sources. It's in copyright limbo, with EA owning the name and some random insurance company owning the dev rights, so it's been out of print for years. But should you... stumble across it, it's marvelous. There's a really active modding community that has made patches to get it working on modern systems. (I got it running on Win7 64-bit without issue. I'd recommend the SHTUP high-res texture pack, but warn against the mods that "improve" the character models, as they take a lot of liberties with the game's look and feel.)
It's from 1999, but I first played it last year and loved the hell out of it. It's really, really similar to Bioshock (or rather, the other way around.) But it relies heavily on RPG elements/character customization: you need to level up weapon classes, psi, hacking, or whatever to actually be able to use them, and the amount of points you get forces you to specialize, so every playthrough can be very different.
To quote Yahtzee, "System Shock 2's difficulty fell somewhere between 'challenging' and 'murderous.'" I first played it on normal, and it kicked my ass the first few hours. Even then, it didn't get easier because my character got a lot more powerful, but because I got a lot more careful. The difficulty serves the atmosphere superbly... I was really on-edge throughout the whole game because even the weakest monsters can kill you very easily. It's like that jump-scare "OMGOMGOMG" terror in Minecraft on hard difficulty, when you're doing your thing and suddenly hear a creeper hissing right next to you. That feeling happens in System Shock 2 all the damn time. It's an enjoyably oppressive experience.
Also, the plot's first-act reveal is something to behold. I mean, I went into the game knowing the spoilers. But despite that, and despite the 1999 graphics, the spectacle of what happened was jaw-dropping. I'd read exactly what was going to happen and when, but it still took me completely by surprise. The scene is masterfully crafted, and the game's oppressive atmosphere builds up to it perfectly. Such an incredible "Ho... ly... shit..." moment.