It would seem that the best papers for pen and pencil would have opposite requirements. You want the pen to glide smoothly and not have the ink feather. But pencil needs abrasion to take off the graphite and an open surface to hold it in place. The notebook papers that I've found the nicest for fountain pens usually allow pencil lines to smudge very easily. So I searched and found a few compromise papers that work decently for both.
1. Ampad Gold Fibre: Works great for pencil. Since I've become used to Japanese notebook papers, my expectations for looks and comfort have been raised. Gold Fibre rates just under the minimum for me.
2. Miquelrius spiral notebook, graphed paper: Evenly pretty good for ink and pencil.
3. Maruman Mnemosyne: Near the top for pen comfort. Allows minimal pencil smudging. I believe this paper is used only in the Mnemosyne line of notebooks.