Absolutely love Zulip. I think they are the #1 open source project out there for many reasons. Here are a few that come to mind:

1. Open source and the commitment to keep it there. 2. The continued technical excellence of the product. 3. Excellent and up-to-date documentation 4. Open to the public development effort that allows public participation (chat.zulip.org) 5. Availability of help from front line engineers and owners as well as the community. 6. Modern and organized UI with many options to tailor it to use case and environment. 7. Excellent choice of tech stack which has evolved to keep up with new technology. 8. An excellent place for aspiring developers to learn not only coding but other skills such as communication and relationship values.