Permalba White stays the whitest but it is very soft out of the tube, it's made with safflower oil.
Any lead white is great, it's thicker and is the fastest drying.
Titanium White is slower drying and has less covering power.
Zink White dries fast but dries brittle.
Ceramic White may prove to be the best white, Holbein makes it. Strontium Titanate
Find this color on a real color wheel.