M143-UF. Zig Cartoonist White Brush Pen Ultra Fine.
Great for dark paper, producing full, semi-opaque to opaque strokes if you go slow and work deliberately, and dry-brush effects if you work more quickly. See M143, for a version of this pen with a larger brush tip.
It takes a little bit of fidgeting to keep it flowing right, but it’s well worth it in my book.