In my opinion, it's Bendy And The Ink Machine and it's sequel, Bendy And The Dark Revival.

Reasons are simple: these 2 games are the only mascot horror games that still scare me not with cheap jumpscares, but with the gameplay and atmosphere. I literally can't play chapter 3 of BATIM at night due to how scary it actually is. The main reason is Ink Demon himself:

"There are so few rules to our world now. So little truths. But there is one rule we all know and respect down here. Beware the Ink Demon. Stay out in the open for too long and he will find you. They say he hears everything. Every creak of the floor. Every rustle of paper. You never know what will draw him in" - that phrase describes the Ink Demon the best. He can appear in a random moment, and all you can do about it is hide or run as fast as you can. That's genuinely scary. And he doesn't jumpscares you, he just appears and starts roaming around for a while, but if he will spot you, then pray that you'll be able to hide in time. No pipe, no axe will save you, don't even bother yourself trying to attack him.

Besides that, the whole atmosphere of the game gives you a feeling that you're not alone, and something might start chasing you in any moment.

I hadn't played Dark Revival yet, but if I'll get myself a good PC, then I'll definitely try it out.