I’ve never really been a major chat user. I think that non-real-time forum-style systems like Usenet, Reddit, or the Threadiverse tend to lead to more interesting interaction.

The chat system that I’ve probably used the most has been phone group text messaging, with IRC or XMPP group chats coming in after that. One place I worked used AIM for a while, and people would use that to coordinate stuff or do water-cooler talk.

If I were going to pick one to use, it’d probably be XMPP, assuming that I could get everyone else to use it. Federated, variety of good native clients for different platforms, support for E2E encryption.