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 us.
iirc there is also a paid hosted version of Zulip but I don't know that for sure.
