Doom 3 - the best horror shooter I've ever played. When it just came out, the graphics looked just amazing. I must say that I never got to finish the game, because I used to be quite scared of it, but I still enjoyed playing it.
What I liked the most about the whole game was the atmosphere. Main character is stuck on Mars, on an "empty" space station. In reality it isn't empty at all - zombies, monsters and ghost-like things are swarming across the dark corridors and waiting for you to come near them. As soon as you enter some kind of a dark room, you can expect some zombie to come out of a wall, breaking a metal plate out from it in the process. If you don't expect it, then the red blood on your screen may come as a surprise to you. Overall the constant dark atmosphere and feeling that something might come out and attack you any moment, are given to the player very well.
The plot isn't the best though. Main character runs across the space station, at first doing what he is told to do (by radio transmission), and later going into the 'hell'. The antagonist is a scientist who is able to control the monsters and even summon them. Pretty much like a wizard. The scientist killed almost all humans in the beginning of the game and then tried escaping with his bodyguard. The bodyguard ended up turning into a monster. As typical for the genre, main character ends up killing all enemies and is saved by reinforcements.
The weapons looked attractive and quite sci-fi'ish. From simple gun and fists to plasma guns and soul-cube. There is a total of 12 weapons which are different and variable. There were some bosses and they were fun to kill, but not too many of them, so it didn't become repetitive. Monsters were interesting and their design was good. All of them looked like mutants though. My favourite was Pinky - a giant half-dog with robotic wheels instead of back-legs.
Summary: Doom 3 can be considered as a very good horror shooter, especially for the time, when it came out. It had amazing graphics for the time. The plot wasn't very good, but not too bad. The weapons, bosses and monsters were quite variable. The game is meant for playing in a dark silent room and is very long - the fun won't end too fast :) . Of course it can't be compared graphically to modern shooters, but it's still good even today.