As someone who grew up in the 90s with a power mac, I very fondly remember the Marathon games. They were the first FPS games that contained a complex story beyond run-and-gun, delivered via the computer terminals throughout the game. In all honesty, I don't think there was any game that came close to the complex story it featured until Deus Ex was released years later.
However, if you want to REALLY feel atmosphere and **** yourself in fright, load up an old mac emulator and play "Pathways Into Darkness" - by the makers of Marathon and Halo, was their first FPS game. Movement was slow, graphics were basic and gameplay was very paced. It also made you **** your pants in fright as a result! It was also near-impossible to complete, at least the first couple of play-throughs due to the maze-like game (it was easy to spend ages going the wrong way) and some really tough puzzles. The monsters are the most cartoony things you've every seen, but that just made them MORE scary! To this day I still can't play it for more then 30 minutes at a time!









