I tend to recommend Opus Magnum as an introduction, it's a lot easier to plan with, nice to look at, and has a somewhat more engaging story than most.
I'm a big fan of all of the puzzle ones, though. SpaceChem had a huge impact on me, my most-viewed YouTube video is this one little clip from January 2, https://youtu.be/dlJmKqi6EEc . But very hard to explain why it occupied my whole soul for a week!
I tend to recommend that people ignore any recommendation to start with a specific Zachtronics game (unless they're getting the recommendation from someone who knows them personally). Different flavors of nerds seem to have vastly different preferences for the games, so it's probably much better to take a few minutes and check how interesting the core mechanic of each one is to you and pick based on that.
Personally I enjoy working with electronics and assembly, and that translated to me really liking TIS-100 and Shenzhen I/O. Meanwhile Opus Magnum wasn't anywhere near as interesting to me and felt like kind of a slog.
I'm a big fan of all of the puzzle ones, though. SpaceChem had a huge impact on me, my most-viewed YouTube video is this one little clip from January 2, https://youtu.be/dlJmKqi6EEc . But very hard to explain why it occupied my whole soul for a week!