I'm putting in a 1'x1' french drain around the side of my house, and I'm
wondering what would be the best material would be to cover the top few
inches of the trench. The soil around here is mostly clay, and it seems
like a better idea not to use clay to fill in the top with, as I'd like
to allow for as much drainage in through the top as possible.

I plan on using 4" perforated flexible pipe, wrapped in a fabric sock,
with the remainder of the trench being filled to within 3 inches of
ground level with some leftover driveway gravel. I might use fabric
around the gravel as well.