Not only is DOOM the grandaddy of the FPS, it is also one of the best, most intense games in the genre it pioneered.
The plot is quite simplistic for an artistic work, but for a video game it's just fine. It is the end of the 22nd Century. The human race has begun to colonize Mars and its moons. But one day, demons coming through portals from hell attacked and destroyed these colonies. You are a marine sent to investigate the attacks on the martian colonies. Surprisingly, you're never actually on Mars; instead, you go from Phobos to Deimos to hell itself.
Despite the simplistic plot, the game itself is incredible. Though you only get seven weapons to mess around with, there are many instances in which you must use all seven, particularly in the last episode, Inferno. The weapons, with the exclusion of the pistol, are all extremely powerful and although they seem similar, they are actually quite different. For instance, one could say the chaingun and the plasma rifle are very similar. However, one is slower and less powerful, while one has limited ammo availability.
The levels are what make the game one of the best of its kind. Unlike the linear, simplistic levels of today's FPS games, the levels have plenty of secrets and passageways. In other words, they have depth. The enemies you face are also quite fun to fight. Few moments in other games can top the first time you encounter a cacodemon, the secret level in which you get all the weapons but have to fight four Barons of Hell, the fight with the Cyberdemon, and having to kill a cacodemon with nothing but a pistol.
What makes DOOM also appealing is its sheer scariness. Particularly in the last two episodes, the textures and the music go together to create an ultimate environment of fear. Seeing a wall of souls moving along a former human facility never fails to send chills into your spine, and the music never fails to deliver.
DOOM's intensity, scariness, and depth are perfect reasons to play the game if you haven't done so already; and because of these reasons the game still shines in the modern day of intense graphics. In short, if you are a fan of FPS games, you need to play DOOM.