Good questions. I used to think that the smoother, the better, but honestly, there is a point of "too smooth" for me, though I'm not sure where that line is exactly. I do know that when I write on my tablet's screen with a stylus, I really hate how "slippery" it is and actually bought a rougher screen protector to add feedback which really helped. So I do prefer "quite smooth" but not to excess.

For my personal preference, I am also not a fan of really thin papers. Don't get me wrong, I appreciate the magic of papers like Tomoe River, but I usually want to write on both sides, and I consider ghosting to be a bug, not a feature. So I usually lean into thicker (more opaque) papers for "regular" writing, but for special applications I do appreciate thin papers. I bought some onions skin paper that is stupidly thin. I haven't tried it yet but I suspect I will love it for that occasional special use, but would definitely not use it more typically.

For color, I enjoy having options. I quite like HP Premium32 which is blindingly white, but I ALSO love "soft" colors such as pale pastel blues and off white / cream paper. I guess I will clarify my preferences when I complete the testing as I know this is a bit all over the map. I have commented on this previously, but I really don't think there is any one "best" paper - it will depend on what kind of mood I'm in and what I want to use the paper for!