Minecraft beta/alpha is the scariest horror game in existence.
There's no game that captures that feeling of isolation and loneliness that minecraft beta/alpha can.
The simplistic and vast world is just freaky to begin with, the world feels dead. There isn't a lot you can interact with compared to modern minecraft, the only food from animals being pork. The silence broken by periodic cave sounds just gives such an unease while playing. I literally almost jumped hearing the sound of grass being stepped on, thinking it was a creeper when it's just a sheep. It truly feels like you are alone and isolated, yet for some reason you still feel like you're being watched. That's what makes it so terrifying.
And, while Minecraft makes new mobs like the Warden, it will never be as terrifying as playing on a Beta or Alpha version of Minecraft.