Pen-Opake is the best white ink there is. The only problem is that they stopped manufacturing it several years ago and nobody seems to have a stash for sale. Occasionally it is mentioned nostalgicly on some art blog or another, but other than there is little info about it on the net or anywhere.
I've been using up the few crusty bottles I procured from the Junk Store on all those ink-and-white-on-paper-bag drawings I've been doing, so I set out to find a suitable replacement. Some illustrator friends of mine mentioned that FW Acrylic Ink is a very opaque and fluid white, so I procured me some and put it to the test... a test it failed miserably.
So I went through my stash of white inks and gouaches to see who had what it takes.