Some of his greatest achievements are:
-The Funtimes. They are very advanced, and harvested Remnant.
-The Sound Chips from the Twisted Ones. While this is a non-canon book, the Sound Chips are in the lore, and make such a high pitch sound they can make illusions.
-The Toy Animatronics had facial recognition.
-While I can’t confirm if the Glamrocks were designed by William before Fnaf 3, but they no doubt took inspiration from Afton.
-Due to UCN being Afton’s hellscape, the way you are able to fend off that many animatronics at once as him is very impressive.