Kokuyo's Campus notebooks have my favorite paper for writing, followed by Staple's Sustainable Earth sugar cane paper line, followed by Rhodia's notebooks.
Generally speaking, I like:
- A5 or B5 pages
- that are light weight, and smooth in texture
- and slightly off white
- while lightly or not at all ruled
- and where ink tends to sit on top of the page, not soaking in. (This both because it combats bleed through, and I think the resulting shading is prettier. Probably not a good idea for lefties.)
If you want a rabbit hole to root around in for ideas on what to use, I'd checkout Fountain Pen Network.