Doom was the first FPS, while Heretic made it a genre.

User Rating: 9 | Heretic: Shadow of the Serpent Riders PC
basically its like Doom, but it takes place around 2,000 years ago. you play as an elf who is on a revenge mission to destroy the "Serpent Riders" who killed your family and your people. much like Doom, there are 7 weapons in the game, but the weapons in Heretic are more well-designed than the ones in Doom. there are also many kinds of enemies in the game, probably about 20 or 25, maybe more. the game has extremely well-designed levels and to top it off, it sports beautiful graphics and sound. i used to play this alot more than Doom when i was a kid because it was in many ways better than Doom, although it was considered one of the "Doom Clones" of the 90's. if you have DOS emulator or maybe even a DOS computer, I HIGHLY recommend getting this game