What do you like to write with? Different writing instruments demand a variety of notebooks. I usually carry a notebook and writing instrument in my pocket - so portability in my “Every Day Carry (EDC)” is a key issue. If you are using a bag - this may be less important to you.
Much maligned, often mocked, but by far the world’s biggest selling pen type. From humble plastic Bics to limited edition Montblancs, there is a ballpoint for everyone. They are generally reliable and robust and will work on pretty much any paper. When I go hiking, I often carry a ballpoint by Schon Dsgn or a Fisher Space Pen. Both are almost indestructible, and can be sat upon with no ill effect.
These work well on most notebooks (can struggle with waterproof papers). However, for a pocket EDC, these can be a little brittle. Beware, inky pocket.
I do like a good pencil. However, one does necessitate a knife, or other sharpener. Hardly a huge inconvenience, but one nevertheless. As with gels/fine-liners above, can snap in a pocket.
A mechanical mostly eliminates the downside of botanicals, in that you click out more lead when you need it, rather than sharpen. Durability varies from model to model. My Rotring 600 has a tendency to stab me in the thigh, which is not ideal.
All fountain pens are not created equally. Many of my favourite desk pens are large, fragile and fitted with big, wet nibs. Quite apart from ink concerns, a double broad nib makes for large letters, causing a pocket notebook to be impractical. However, there is a whole class of fountain pens ideal for the job. Pocket pens. For me, the King of the pocket fountain pen is Kaweco. I have a collection of Kaweco Sports in all manner of materials, from lightweight acrylic to chunky brass. These can be fitted with nibs from extra-fine to double-broad and take cartridges that come in a myriad of colours. Kaweco also produce the Lilliput, a tiny pen, that when posted, makes for an elegant, minimalist writing instrument.
I change my EDC all the time and in common with many stationery fans, I have a collection way too big for my needs. The writing instrument that I want to use will inform my choice of notebook and vice versa.