Peter Pauper Press Essentials. It's been my go-to since I started bullet journaling five years ago. Pen test
My favorite's the Peter Pauper Press Essentials. I use the dot grid, it also comes in lined and square grid.
My favorite's the Peter Pauper Press Essentials which is always between $9-12, also comes in square grid and A4 & A6 sizes if those are your preferences. A lot of people get the Leuchtturm1917, the A5 size is always around $20, personally I can't stand that brand because I consider the paper quality to be terrible for that price range but not everyone uses the same materials, I find it to be terrible with gel pens like the Pilot G-2. Another popular brand is Scribbles That Matter, also around $20, I can't vouch for them though.