My notebook of choice is a... sketchbook: ArtCreation, from Royal Talens the Dutch brand. It's 120 gsm smooth paper, plain, great for sketching (& even for light watercolors) and for fountain pen writing. Most often, I'll be using the A5 size, bu the're available in various sizes. Also, the sketchbook will stay flat when opened.

Before the ArtCreation sketchbooks, I used to use Leuchturm 1917 and Seven Seas (my favorite, but I wanted thicker paper).

I don't use cover or case.

I also always carry a pocket A7 Clairefontaine notebook, for quick notes and stuff.

Lose leaves of paper: Clairefontaine or Rhodia which are obviously readily available here in France and work real nice with fountain pen. l will also re-use all the back of letters, envelopes, old sheet of papers (drafts), and so on.

I will cut them to size :

• A5, to fit my Filofax-like 6 rings binder.
• A6, to fit my Zettelkasten-like note-taking system as I don't want to use Index cards: they're too thick and waste precious storage space (plus, I like recycling old sheets of paper)

Maybe worth mentioning next to the paper, there is the ink:

• De Atramentis Document Brown, for sketching, waterproof.
• Platinum Carbon Black, for writing, waterproof.
• Pilot Kon-Peki (blue) for writing, non-waterproof or I will use the basic
• Waterman Serenity Blue, Pelikan Blue, for writing, non-waterproof.
• Plus a pen loaded with a red and one with green (both Lamy ink)