I use a leuchtturm1917 A5, dotted paper. (2) Paper quality can vary, if you use a proper pen you’ll want to go for the thicker paper. You can get better journals on Amazon (cheaper and more pages), but I am lazy and can’t be bothered to number the pages myself. I also attach two pen loops, for a pen and a mechanical pencil.
1. https://en.wikipedia.org/wiki/Bullet_journal 2. https://www.leuchtturm1917.co.uk/edition-120g.html
Isn't it confirmed by studies that writing notes by hand on paper reinforces learning more and better than others forms of notetaking (e.g. typing)?
Then I have project notebooks, where each page is dedicated to one project. I only take it out after spending some time in the mind-map kind and I have made enough progress that it's worth writing in the project one. This notebook is written as documentation for an imaginary third party. It's useful for sharing with others and more importantly myself in the further future.
I use this system for work and personal projects and while I don't too often have projects running long enough for this to matter I regularly refer to my old solutions and thoughts. It has definitely prevented me from walking in proverbial circles.