Nielsen's usability heuristics, arguably his most impactful work, still hold up surprisingly well after more than 30 years. It's hard to understate the impact he has had on my own career, as well as the UX field as a whole (together with Don Norman, of course).

Also sobering to read how much of his career seems obvious in hindsight, but also was shaped so much by randomness and chance (such as taking not taking the job at Apple).