This is my biggest weakness. I haven't worked on anything where I made a substantial contribution that I'm proud of. I've been working as a programmer for nine years.
Three years in a consultancy implementing solutions based on a third-party framework. The framework was terrible, so the best we could do was to paper over its cracks. We made things much better, but it was still very poor compared to normal software.
Two and a half years at a small company, working on their legacy codebase. We did innumerable small cleanups, bodged in new features, and did one bigger architectural change. Even the big change was the sort of thing that would only take a week in a sane codebase.
Two and a half years in an agile consultancy. Many short projects, mostly building technically rather simple things, but focusing on teaching the clients how to be better at software development. The one technically complex thing we built was mostly only complex because some plonker settled on a microservice architecture using a new messaging framework before I took over. I think we did a good job, but I can't justify the central design decision of the project!
Those companies were all agile, with pairing, collective code ownership, and self-organising teams. Even where we did something good, it wasn't my work alone.
One year so far in a hedge fund. I've built several small tools and features, most of which haven't ended up being very useful. The two bigger things I've done have mostly been about wrapping sophisticated third-party libraries in an application which integrates them into our environment - I'm a plumber.
Everything I've done has been what seemed like the most useful and sensible thing to do for the client or company at the time. None of it helps in interviews.