I've tried many different ones.
- White Prismacolor pencil. Doesn't always show up very well on white paper, but works great on toned or colored paper.
- White Uniball Signo pen, which has surprisingly opaque ink. I find myself using this or the Prisma pencil the most. The Signo is great for fine edge highlights. On the downside, it doesn't flow evenly and as a result can look splotchy.
- I've used Deco Color paint markers but I've found them to mix with the other inks on the page. I want to try Molotow and Liquitex.
- Faber Castell Pitt artist pen, white. Works pretty well but the tip is much broader than I like. These are popular in the ID community.
- White paint sharpies were horrible.
- In school we used this white paint (with a tiny brush) that worked well and was super opaque, but I hate using brushes. I can't remember what it was called.