Dye-based vs pigment-based inks
Most fineliners use dye-based ink: bright, well-behaved, but water-soluble and not lightfast. Leave a dye-based fineliner line in sunlight for months and it fades. Add a wash of water over it and the lines bleed. For everyday journalling, this does not matter. For art, mixed-media work or notes you want to last, pigment matters.
The Uni Pin
The Uni Pin is pigment-based, waterproof and lightfast. A 0.1 or 0.2mm Uni Pin under a wet watercolour wash will not budge. The ink is also fade-resistant, so your notes from five years ago will look like they were written this week. Available from 0.03mm to 0.8mm.
Artline fineliners
The Artline 200 and 220 series use permanent-type ink that resists most water exposure, though they are not quite in the same archival class as the Uni Pin. They are well-priced and widely stocked - a good choice for annotation and study notes where you do not need gallery-level permanence.
For mixed media
Waterproof: Uni Pin. For general journalling: Staedtler Triplus or Stabilo Point 88. If you are doing watercolour illustration or botanical drawing: Uni Pin at 0.1 or 0.2mm to outline, then lay the wash over it.