If it means design as in, the actual design of the 3d model, my pick would go either to Ennard from Five Nights at Freddy's: Sister Location, or the Scrap animatronics (Molten Freddy, Scraptrap/Afton, Scrap Baby & Lefty/L.E.F.T.E) from Freddy Fazbear's Pizzeria Simulator.

The reason for those is as follows:

Ennard's concept of being made from the parts of Circus Baby, Ballora, Funtime Foxy, Funtime Freddy & Bon Bon is just really unique. I don't think anyone has ever done this before Scott ans he just pulled it off very well.

The Scrap animatronics designs have always been very interesting to me. Scott seems to have put a lot of thought into their designs, seeing how Scrap Baby is mostly made out of parts she presumably found inside Circus Baby's Entertainment & Rental (Sister Location) and Lefty got an entire blueprint explaining its purpose.

I just wish I could ask Scott about the details that I can not explain, such as everyone's suit being different all of a sudden or Molten Freddy's eye changing color constantly.