I typically use Pigma pens, but also have a collection of Tsukneko "Fabrico" acid free markers with dual tips which work well, though have a broader tip than the Pigma pens. Both have held up to washing and do not bleed as long as you write with a light touch. I would recommend always doing a test/draft on a scrap of the material to be used for your label. I also lightly press the quilt label when finished to "set" the ink.