Depends on the size you're looking for, but there are plenty of great choices for fountain pen friendly notebooks!
I'll start by gushing a little about Tomoe River paper. It's impossibly thin - think tracing paper - but is extremely resilient, and is one of the best options for fountain pen users around. It resists unsavory characteristics from even the most foul-tempered inks, and brings out the shading and sheen in inks that typically don't feature either on other papers. It's found in the popular
Hobonichi day planner, designed by
Itoi Shigesato. For general purpose notebooks suited for use as a journal, my top recommendation goes to
Nanami Paper's Seven Seas notebook series. It's A5 size, which gives plenty of room per page to jot down whatever crosses your mind. They go out of stock almost instantly, so availability is an issue, but I love mine and can barely imagine using anything else.
Tomoe River does exhibit what we call 'ghosting' or 'see-through', where you can easily see what was written on the other side of the page, but it never really harms legibility. I was pointed towards the
Taroko Enigma today, which uses a heavier form of Tomoe River intended for industrial printing, that may cut down significantly on this. But, I can't speak on if the 68gsm paper performs as well as its 52gsm sibling.
Next up is
Midori MD. Another A5 size notebook, Midori MD paper is a strange beast. It is much thicker than most other papers, a few slivers away from classifying as card stock. Writing on it will bring out the tooth in even the smoothest of pens, which some users don't like. But, I love my MD all the same. It handles linework with a distinct crispness, and its cream colored pages add a nice, warm touch to any ink's color and shading properties. I use it more for a project planner than a day-to-day journal, but, it might stick with you!
I would be remiss to neglect Rhodia, though I find many of my pens and inks skip on it semi-regularly.
They have a ton of size, shape and binding options, including spiral-bound and their luxurious leatherette-Webnotebook. The paper is coated to provide extremely crisp linework, which comes with several drawbacks. The first one most users run into is the elongated dry times, which average ~10-15 seconds longer than other brands. The second is skipping, which won't happen with
every pen and ink combination, but has tripped me up on several occasions. Due to the way the paper is coated, the surface can become too slick and leave the pen unable to lay down any ink mid-stroke. Your mileage may vary, but I wouldn't recommend it if it wasn't so excellent the other 95% of the time I've used it!
There are plenty of other options to look at from companies like Franklin-Christoph, Quo Vadis, and Maruman, but I have the most experience with Tomoe River, Midori and Rhodia paper. They're all quite different, which makes the choice even more personal and a part of the whole writing experience!