I'm also a hand quilter, I use General brand pencils. Washable graphite on light fabric, white chalk pencil on dark fabric.

I do use mechanical pencils, too. Which I use depends on what I have on hand and how smoothly they travel over the fabric I'm using.

I use the Bohin mechanical pencil with white 'lead' for dark fabrics and a plain out mechanical pencil with #2 lead on light fabrics. That works fine for me as long as I don't get too heavy handed when marking. Bohin makes other color 'leads' for their pencils but my LQS does not carry them. Another shop further away does but I'm not there very often and have forgotten about it when there the last couple of times. I do like their products very much.

And I mark as I go as well.

If I don't need a fine line, I swear by old school tailor's chalk. It's been around forever because it works! You probably even have some already, I think it breeds in sewing machine drawers when nobody is looking.