Permalba Oil White stays the whitest and it is very soft out of the tube, it's made with safflower oil.
Lead white PW:1 is great, it's thicker and is a courser grind, it's also the faster drying.
Titanium White is slower drying and has less covering power because of it's fine grind.
Zinc White dries fast but dries brittle, lead and zinc is a good combination.
Ceramic White may prove to be the best white, Holbein makes it. Strontium Titanate
Real Color Wheel main page.