Story wise, I'd go with Springtrap for what his origins are. Gameplay wise, he's not scary at all. He just strolls into the room like "Hey, bro, you look tired. Wanna take a break and let me take over watch?" Maybe Foxy or Mangle (or whoever did the bite of '87) as second place for story.
For appearance and behavior, Toy Chica and the fuckin' Puppet. Toy Chica for some reason takes off her beak and removes her plastic eyes for nighttime prowling. That toothy, empty-eyed face of hers is something you don't want to see coming at you through a dark hallway in real life unless you're looking for constipation relief. It goes past the uncanny valley and digs straight to hell. The Puppet is right behind her digging away, too. It lunges at you in FNAF 2 and stares and stares and stares both in a cutscene in 2 and as a hallucination in 3. Your personal space belongs to him during those moments. Old Bonnie doesn't have nothin' on these two, even without half a face.
Personally, I'd watch a Saturday morning cartoon based on the friendly interpretation of Foxy. In FNAF, he's shy around the other mascots and just wants to meet a new friend. Too bad the player dies of a heart attack, if he makes it through the door, before he can introduce himself. In 2, he wants to give the player a big glomp-style hug, but doesn't realize he weighs so much that jumping at someone like that will crush their ribcage.