Relative to my technical skills.

One of the PRs was some data visualization code in the Elm language. They explained the solution so beautifully, showing how the problem lay in how the Elm compiler compiled to JS and how JS managed memory. It just blew me away that this person did this in an evening, with no experience in Elm or front-end developement, and that it would've taken me days to be that precise. And they weren't even out of college yet.

Edit: This was pre-LLM, where research and reading were a big part of the job