Were using Zulip. Before that we used Mattermost but we migrated to Zulip because of its (in our opinion) superior conversation threading model and never looked back. Mattermost, Slack, etc. all work fine when you work on a project basis but when you work on a lot of short lived operational issues (for example "Issue with invoice XYZ" Slacks conversational threading model breaks down quite fast. At least it did for u…
We used mattermost in the past... maybe it's different now, but I remember the UI being frustratingly odd. Definitely didn't have the pleasant usability of slack.