DOOM, by far.
I don’t know another game that has so deftly managed to tie everything about it - its mechanics, its thematics, its aesthetic - so cleanly together, and all with one single gameplay gimmick. The glory kill system does something that even the original DOOM struggled with at times, it keeps the player aggressive, on the move and always in the thick of the fight. And that Id Software managed to wrap those mechanics in a story that straddles the line of sincerity and pure Evil Dead style buffoonery is deserving of praise - so is the fact that they managed to give a silent protagonist far more character than most meat head white dude FPS protagonists.
Also the soundtrack, aside from being so damn good, is another example of how singular this game in its focus. The dirty synth infused metal soundtrack ties directly into the aesthetic of the game - the H.R. Giger-esque marriage of flesh and metal - and the pacing is pitch perfect. The composer, Mick Gordon, stated how he would watch gameplay of DOOM and setup a metronome so that his composition would match the tempo of the gameplay, and it shows. The soundtrack turns every combat encounter into a macabre ballet of blood and guts.
Honestly I could talk about this game for hours, I love it - more than the original DOOM, even.