The slow-burn dread of realizing what's happening, hands down. That moment before the full outbreak, when people are noticing strange news reports or weird bites on their neighbors but the authorities are telling everyone it's fine—that's pure tension. It's the collapse of order that gets me, not the gore. The way a supermarket turns from a mundane chore into a battleground for the last can of beans.
I'm less scared of the zombies themselves and more terrified by the living who turn on each other. The real horror is in the human choices under pressure, the moral compromises. A story that spends time on that societal fracture, like the early seasons of 'The Walking Dead' or 'World War Z' the book, gets my heart racing far more than any jump-scare horde scene.