Just as a list, the other animatronics are Toy Freddie, Toy Chica, Toy Bonnie, Balloon Boy, Shadow Bonnie (I mean j guess by technically an animatronic), endoskeleton, Spring Trap, Phantom Balloon Boy, Phantom Freddie, Phantom Marionette, and Phantom Freddie (again the phantoms aren't technically animatronics, but they were freaking scary).
There were two things that made Spring Trap the scariest to me. The suspense of the whole game made Spring Trap seem like the devil himself, and the fact that there was actually a maimed human inside of Spring Trap.
None of these, but for me, it's Springtrap. He has a maimed, decayed body inside, is controlled/possessed by the killer, and the way he jumps out at you. All the others scream and jump at you to kill you, while Springtrap just stares at you and then slowly leans towards you... It's more creepy to me, just because of how deliberate it seems and how slow it is