The best I've come across are the dry erase markers with the eraser on the cap, can't currently think of the brand. Those paired with a sharpie are the best team. If you haven't used the sharpie trick on a slate, you are missing out. Write stuff that doesn't change often (director, camera, date for the day, roll) in sharpie, and you can easily erase the sharpie by tracing over that writing with the dry erase and then erasing them both. The dry erase formula acts like a solvent on the sharpie, making it removable. All it takes is one shirt rub to smear or erase a majority of your slate, so if you aren't using labels, this is the tried and true method of retaining a majority of information. Of course it is much harder to remove if you leave a slate sitting around for months without removing the writing.
ALSO, how well slates react to markers really depends on the brand on slate. It seems the cheaper slates roaming around tend to stain much easier than say a denecke smart slate. Just my two cents. Hope it helps!