經典黃色螢光筆（單支）

The chisel tip is the best part, flipping it sideways lets me highlight a whole line in one pass and the narrow edge works for margin notes. Yellow's bright enough that it doesn't wash out printed text.

The ink dries almost instantly, so I can highlight a whole page and flip it without any smudging. I also like that I can switch between a thin underline and a wide swipe just by turning the tip.

That chisel tip is the reason I keep buying these—flip it one way for fat strokes across whole sentences, the other way for underlining single words. Yellow's bright enough to spot instantly without bleeding through my notebook pages.