Normally I'd say black since it's my favorite color. However, the black seems plain and boring to me TBH. Unless you want something like that, I suggest going for the gold one.
The question is tho, is this case brand good for protection of the phone? If so, my personal pick is gray
I like the gold one and the silver one equally, so I'd recommend picking those two. The gold makes it contrasts from the black very well, but it stands out too much. The silver somewhat contrasts with the black very well and stands out perfectly, so it's basically just your preference there. I wouldn't choose the black case. Seeing as the color of the phone is white, a black case wouldn't fit the style well. Light colors look nice with light colors, dark colors and light colors don't usually mix well together.