I use a Moleskin Cahier notebook slipped into a Yellow Birch Outfitters PocKit Pro to keep it from getting destroyed. I like the Cahier notebooks because they have perforated pages that I can tear out and leave notes stuck to places with glue dots.

If you’re just looking for the most durable notebook though, something like the Rite In The Rain spiral bound notebooks would work. Just my opinion but for pocketability’s sake I would advise against a hard case since it’ll be bulky and won’t conform to the curves of your body.