The answer is probably a focusing screen that is well matched to a certain lens, rather than a certain camera body. A screen that is the brightest for one lens will not be the brightest for a lens that is significantly faster or slower.

Also, the focusing screens that are the "snappiest" (i.e. that show the shallowest depth of field) are usually not the brightest. For example, Canon's newer type S screens show the depth of field at f/1.8, but they are about a stop darker than their standard screen, which show D of F at only f/2.8.

So, I would say the answer for you is the Nikon body that accepts the largest variety of useful focusing screens.