This game is one of my most beloved gaming experience, the gameplay is superb and the setting and atmosphere is completely unique it has so many different inspirations if you are an horror movie fan and played survival horror games, you'll find something in that game that echoes some memories.
For me it is on par with RE1 (director's cut) and RE4 as a continuation of the formula, the gameplay is the key here, I put this above everything else when i rate my time with a game, especially with a horror title, due to the inherent flaw of this genre : once you get familiar with the game, it cease to be scary, so you are left with the atmosphere, setting, story, OST and more importantly the gameplay.
RE1 is amazing to this day because it is very precise and running around raising up your shotgun to blow zombie head off never gets old, nor does it fails (a particuliar flaw of the remake for me) you replay the game many times for this reasons.
RE4 is the similar, don't tell me it's story is what kept you playing it times and times again (for the memes it is) gameplay is king so does the pacing of the game.
The Evil Within is all of that, with added traps you can set, defuse or avoid when used against you and more importantly stealth mechanics (one hit kills ala Tenchu conserves ammo and don't alert all the enemies) a pseudo RPG system for leveling your character (currency found on dead enemies or by exploring that can be spent on either your stats/capabilities or weapons, choices again!) and a reward for each hidden keys you found during the game in the form of a randomized loot (could be ammo, health, currency etc), you can have double HP and more powerfull guns to gun everything down or be more agile and have more traps to flee encounters and hide if things didn't go as planned, letting you try again after the enemies ceased to search for you.
This is more freedom given for you as a player to tackle each situation, more control for you to play stealth mostly or gun blazing by leveling up specific stats that emphasize your playstyle, and a huge boost for the feeling of player progression as you complete the game.
They are so many cool ways to kill many enemies all at once by using smart strategies or setting traps it will reward you the more you experiment, say you got 3 guys running to kill you, you can shoot one in the leg (he falls) you can use a match on him to set him on fire and killing him, if timed correctly you have effectively created a fire trap when the others got so close to maul you, you took a risk and now they are all dead, just using one bullet and one match while avoided taking damage, then again you can just shoot them and spent precious scare ammo and health recovery item for 3 goons.
The whole game is like this, with dozens of enemies thrown at you like (RE4 in the cabin) there is explosive barrels, electrical hazard or traps that can be used in the environment if you find them, superb action moments, memorable boss, classic puzzles, the whole game is full of sandbox type encounters for you to enjoy it's gameplay, it often feel insurmountable but you got so many options that there's never just only one method that work.
Then you got the story and the diversity of locals, periods and enemy types to never let the player feel at ease or accustomed to a situation.
For me personally, it's the only game that merged gameplay, story, horror and depth in its mechanics this well, others got a much better story but flawed/basic gameplay, others would have fun as hell gameplay but a shit setting and a near non existing plot, none of them got the balance quite right as the Evil Within does.