Basically I mean... what writing instrument do you use to write kanji? Do you use a mechanical pencil? Do you prefer a calligraphy brush? What is the best way for you to write kanji so you feel like you are being the most accurate.
Same, .5 mm lead pencil or liquid ink pen for most situations. .7 mm is fun if you're writing larger and have calligraphy experience, since it's enough lead/ink surface to actually add some angle and direction to your strokes.