Fountain Pens

Midori MD pads with one of their covers work well and aren't too expensive. Available in A5 and A6 if I recall. Otherwise as already mentioned I like the 90gsm Rhodia or Clairefontaine notebooks. The 80gsm Rhodia paper (found in a lot of their pads) always feels like the backside of the paper has less coating on it and not as enjoyable.

I've gotten good mileage out of Rhodia and Claire Fontaine, and my sibling swears by Tomoe River. Do not try Moleskine, I have gotten more feathering on those notebooks than on any paper I have ever used. I think they're designed for pencils, or something.

Fair disclosure, though, I usually just use cheap composition books, and find a pen/ink combo that looks good on whatever I happen to be using. I only use nice paper if I particularly need it for a given pen or ink, or if I am going to show someone else what I've written.

Midori is lovely, especially for fancy inks - shimmer and sheen are more visible in my MD notebook than on Rhodia and Lechtturm papers that I’ve also tried. My favourite MD notebook is A5 grid additionally divided into 4 squares, works great for journalling and for the Goldlist method. No idea how this particular MD notebook is called, I buy it from Stilo e Stile.

I've really enjoyed my TūL notebooks. They don't seem very common in the FP forums, but they're easy to get a hold of in stores (in the US), come in many sizes, are expandable, and the paper quality works very well with all of my inks this far (it is not particularly quick to dry, but also not likely to feather).

I like my Hobonichi Techo planner. The paper is thin, so it ghosts a bit, but it doesn’t bleed. I think they also make plain journals, and it’s not uncommon for Hobonichi fans to buy an extra planner to use as a journal.

I use a Hobonichi techo planner and grid notebooks. I also use Midori grid notebooks. No feathering on both, not too glossy so my nib doesn't "skate" on top. I like Rhodia pads (some of the notebooks feather). Clairfontaine is lovely smooth paper but my pens usually "skate" on it and I can't control my handwriting on it, but YMMV.

@kalanggam As others have said, the Hobonichi's are great with fountain pens, or if it's more of a pure journal you're after the Luechtturm1917 Bullet Journal performs excellently.

I originally bought into the hype for Moleskine, but their paper seemed very low quality when I tried writing on it in person at the Moleskine shop in NYC.

Muji notebooks came very highly recommended for their price to performance ratio, and they're the real deal. I much prefer spiral bound notebooks that lay flat like this one, but they also offer a different kind of paper I haven't tried yet.