The classic, The Original, The one, The only DOOM
If you really don't know what this is about, first, whack your head very hard against your desk now, when you come round, get onto id software's website and buy a copy
As most people already know, you are a marine trapped on mars fighting a tonne of hellspawn through 3 episodes, this title later saw 2 sequels which continue the honour of this great game. Even after all these years, people appriciate this game. The graphics are great for it's time, while it looks a bit aged now, there are loads of sourceports that either make the game playable on newer OS's which don't support DOS, or give the game a complete graphics/sound re-do and add some new features, if you want to stay close to the original, get Zdoom, if you want 3D models, high-res textures and a more modern feel, then doomsday will do for you, try a few and see which you prefer. If you want to play online, there's Skulltag, Zdaemon and Odamex, i, myself prefer Skulltag because of it's new content and mod support.
Gameplay:10 proves you don't need fancy physics, reloading, prone and whatever else you may see in a modern FPS
Graphics:10 depends which source port you use, jdoom with all the models, textures and effects look stunning, though anyone who plays doom couldn't care less about the graphics
Sound:10 while the sound effects are pretty generic, they sound crisp and do the job rather well
value: 10 4 episodes (if you own ultimate doom) and 5 difficulty levels, and some nice online with Skulltag/Zdaemon (though Zdaemon sucks IMO) you cannot go wrong here