I tried tons of things. I find that a cheapo notebook with lots of pages is best for me. No themes, no rules, just ANYTHING can be written. I do write the date at the beginning of any entry, be it journal, planning, etc, because that really helps with looking back and making it useful. I also try to
skip lines or open space because inevitably I will want to go back and write additional notes or addendums when reflecting.
If most of what you write, plan, and log is text, then a regular straight-line ruled notebook is probably best.
If you do a lot of designing then a bullet journal (one with tons of dots) or a graph paper journal might work better.