I found the Apica A4 notebooks to be really nice, but a bit on the pricey side given how many notebooks I go through. I found that Kokuyo has their Campus notebooks in a B5 size that's only slightly smaller than an A4. And, the nice thing for me is they have a 6mm rule instead of the 7mm rule, meaning I can get more text on the page.
The Campus notebooks are a bit lighter weight of paper, but they work well, and are fountain pen safe.
Maruman also has some nice spiral bound notebooks that are semi-B5 in size, and have perforated pages. I was looking for an A7 from them, but I could only find ones that had a spiral across the top, instead of on the side.
