I finally shelled out 11$ for the Copic Opaque Ink, and I'm glad I did, as it's the best white ink I've tested so far. The way I do these tests is I dip my brush into the ink, and continue to make marks until it runs completely out. This Opaque Ink is really that- a very opaque white ink that covers well. I'm really pleased with it, and will probably be using it for corrections from now on. It''s too thick to be used in technical pens, but it's just right for brush application.
Beneath it is the Mitsubishi white china marker, which isn't intended for correction, but I thought it might be neat for effects like smoke on an all black background. Unfortunately, it's a little blue for me, so I probably won't be using it after all.