I'd guess the Utah teapot, it's been around since 1975. Cornell box is from 1984, and test scenes for materials are in some way a pretty recent thing, at least

I'd guess the Utah teapot, it's been around since 1975. Cornell box is from 1984, and test scenes for materials are in some way a pretty recent thing, at least how they are used now; to me the modern versions started with Maxwell Render.