Go make something!
It's hard to have just one favorite but here is one of mine, an SD60M. It is a Proto shell on a modified Athearn drive. I fixed the major dimensional errors on the shell, mainly the area just behind the cab that was too low putting the grills in the wrong places. I also fixed the seam line on the nose. The Athearn SD40T-2 drive was stretched to the correct wheelbase and the trucks narrowed to prototypical width. It was fitted with a Athearn Genesis motor and NCE decoder and LED lights. I added some details on the pilots and, since I started with a decorated shell, fixed the paint and decals I'd messed up while doing modifications.