What I tried so far:
P3 Morrow White - Pretty decent, but needs a (light) grey base
VGC - White - takes a few layers, but can look ok (Not if you jump too much in shades, so darkgrey --> white = grainy, light color --> white = ok)
VAC - White - pretty much the same, grains are smaller, but because it's thinner it needs either a light grey base, or quite a few layers
GW - Base - White - Covers even dark colors pretty good (even black) turned out to be great, but dried out in the pot in about 6 months , so it is sensitive to it.
Tamiya - Flat White - really lovely white, but best used with an airbrush. Covers well with a brush too, but to me all Tamiya feel a bit weird.
Rackham - White - Not produced anymore, covers quite good, but unless diluted enough it's grainy as hell.