The kind that you are/were passionate about and can speak knowingly about the techs you used in the project. In rare cases, your side project is addressing one

The kind that you are/were passionate about and can speak knowingly about the techs you used in the project. In rare cases, your side project is addressing one of the businesses needs directly, giving you domain knowledge. More likely is that it does not. At this point, the value-add of that project is that is improves your understanding of the techs you used. Can you talk about the pros and cons of using Go/React/Angular 2/techBuzzwordDuJour? That speaks to your passion and creativity, and stands to improves the knowledgeable of the team that hires you. tl;dr: Do something fun that you enjoy. The rest takes care of itself.