I used 1 x 8 and 1 x 6 pine. Unpainted at the moment, but probably a dark color down the road. I used 1 x pine as I had several pieces left over from work around the house. Different sizes because I have some pullout items under portions of the table that the 1 x 8s didn't allow clearance for.
One issue I had is angle cuts on the 1 x 8s, my miter saw isn't big enough so had to finish cuts with a hand saw and lots of filing. The 1 x 8s give better coverage however, especially with 2" foamcore + 3/4" plywood + 2 x 4 framing. The masonite many use on the forum does look really good and I would recommend that as well.