Perfect DarkPerhaps the most difficult FPS that I've ever played, based on normal difficulty. Not only does the shooting get quite difficult when enemies start toting grenade launchers, OP assault rifles and
cloaking devices, but the objectives are often puzzles, making them a gameplay challenge in their own right.
The original N64 version suffers from quite a bit of slowdown. It would have benefited from being a Gamecube game, but we all know how that story ends...
There was an Xbox 360 port via XBLA. It runs at 60 FPS and features modern control schemes, but IMO the new textures look really uncanny-valley weird.
Operation Flashpoint: Cold War CrisisIs it arbitrary and unfair? Yes, so is war.
It takes a certain appreciation for clunky, weird PC games to get into this one. On one hand it's an amazing military simulation, featuring tons of weapons and units in a massive open world, with a extensive campaign covering the stories of riflemen, tank commanders, helicopter pilots and black ops commandos...
But on the other hand, it's a non-Q3-based PC game from the early 2000's. There's plenty of bugs and glitches, the controls are not always what you expect, the terrain (and characters, and buildings, and animation, and particle effects, and some of the vehicles) look like ass, and the voice acting is pretty silly.
But it's a really good game, if you can see past that. The battles are very tense and meticulously detailed, requiring more planning and discretion than quick shooting. You won't know where your enemies are until they start shooting; its rare that you'll get the drop on your foe.