The game that properly kickstarted the FPS genre, DOOM has a deserved place in the annals of gaming history. Play as Doomguy, a soldier in the wrong place and the wrong time.
DOOM is probably one of the more defining games of my childhood. The core gameplay is stripped down (mostly due to technical limitations of the time), creating a unique experience. Be aware the jump from Ultra Violence to Nightmare is quite large, be prepared to replay easy maps now turned impossible. I had much fun trying out mods, there are countless experiences you can try out. 8/10
Doom has countless mods available for the platform. From high quality experiences to Terry WADs, there are plenty to choose from. Be aware that to mod, you will generally encounter WAD files. PK3s do exist, however they are usually rate. For WADs, you will want to distinguish between IWADs, which contain the base game data, and PWADs (Player WADs), which contain the mod data.
To load a PWAD, it will depend on how you run the source port in the first place. If you are using the command line, use the -file parameter. If you are using the standard launcher, drag the WAD over. The best GUI way would be to use a custom launcher such as ZDL, which allows easy changing of source ports and multiple PWAD loading.
Doom is unique in that since its source code was released quite early, there are plenty of source ports to choose from. I'll break it down into thse categories: