It's said that white diamonds are the cleanest and most white BEL. They're made from het Russos.
I bring it up in every thread that mentions this, but I have never seen a clean one in real life, they all had a yellow dorsal stripe.
In my experience, Lesser or Butter x Lesser or Butter seems to be the whitest, but there is a chance for bug eyes. Lesser or Butter x Mojaves are normally white, but I have seen a few that yellow out.