In my experience, the whitest snakes are pure white BlkELs, all-white Pied Lesser (but these have microphthalmia), and the White Wedding variant of Pied Spider.
Every adult "white" BluEL I have seen has had a faint pigmentation to it so that it was more cream white or milk white rather than the stark, pure white of the previously mentioned.
I also had a Ivory Butter Pastel that was quite white, but again, it was more a cream white than stark white