I have been using Zulip only for three months, but I can't help wondering how I managed to get anything done without it. Everything about this chat is so thought through that I consider it essential to stay productive. Organizing all conversations into streams and topics makes it clear what each discussion is all about, so you can always mute everything that's irrelevant to you at the moment, and focus on important things. I didn't initially appreciate the full-text search, but now I regularly use it to find all the places where a particular subject was discussed. It's almost impossible to lose something with Zulip.
In addition to that, Zulip for macOS is a total eye candy. I like the minimalistic design, the colors, everything about it! I couldn't have wished for a better app. I also have Zulip for iOS, but I don't use it as much. I think it's kind of difficult to take advantage of all the great Zulip features on a small screen.
Zulip is built by keeping productivity in mind. Each conversation in Zulip has a topic. This allows you to take part in more than one conversation at the same time. This also make catching up on conversations efficient as compared to cSlack, Mattermost etc where you have to scroll through hundreds or thousands of messages to figure out what is going on. Zulip is the most active open source chat application in GitHub and has 350+ contributors from all across the world.
