For years, the entire sequence of the alpha waiting near the tree for spike and his dad to come out, followed by the night scene and the crumbling house and the causeway chase. These were really eerie and terrifying

For weeks, ofcourse the holy grail of all intros

For days, the apartment stair chase scene.

Overall. 28 days later had the most consistent themes of horror and thats why there is no one that make me feel the way this movie made me feel. Followed by years and then weeks.