I just buy store brand or brands aimed at students. I obsess over a lot of things but I don’t get expensive notebooks. I’m just not paying silly money for a block of paper to write throwaway notes.
There's also an Italian mill that produces outstanding and relatively cheap paper and notebooks, Fabriano.
Clairefontaine: Same question. I know it has a great reputation for fountain pen ink, but I had bad experiences with it. In a sense, it's too good. Line strokes tend to be thinner than most other papers. I can't write on them with my F or EF nib - I get a lot of skipping. Others have had the same experience - the paper is just not at all absorbent (hence absolutely no feathering).
I had a similar experience with Maruman Mnemosyne.
(Just my opinion - I know there are folks who love it).
I often use Figurare - it's similarly priced, although some of them feather more than others. I just tried Denik, and it's decent. Not as smooth as Clairefontaine (nothing is!), but handles the ink well enough.