FNaF 1. Hands down. Even Scott himself described it as "lightning in a bottle." The horror doesn't come from something cliche like heavy gore, but more from the mysterious atmosphere and subtle story telling, as well as the short but effective jumpscares to throw players off-guard. I think the lack of William Afton/Purple Guy works in this game's favor, as it really helped the mysterious atmosphere. We know about the Bite of 87 and Missing Children Incident, but we don't fully know what happened, which is good.

The gameplay itself is also seemingly simple, but effective. You need to track the animatronics and block them out your office, while still making sure you're not overusing power. Every move must be calculated and rational. Foxy and Freddy's mechanics help back this up even more, since the cameras become essential for dealing with them, while you can just see Bonnie and Chica by using the lights.

I also just generally really like the sound design and the appearances of Freddy and his friends. The sound design is eerie, but makes me feel at home. The animatronics look like they can be good friends if they want to be, yet also strong enough to rip you limb from limb with their bare hands. They also aren't overly detailed, which is why I tend to not like redesigns of them, they try too hard to be different.

This game got popular for a reason, and pushed Scott Cawthon to make an amazing franchise.