Many of our developers started using Slack back before Teams existed as S4B was junk, but we’ve seen a huge increase in usage of Teams since we started working from home. Not sure if any of those devs have moved over; we’re hoping they will as they are using the free (read unencrypted) version of Slack.
Slack and Flock were pretty similar. I think we spent more time with Flock but it could just be because we used that one second.
Teams is a whole different beast, and while it doesn’t have the same kind of “channels” that the former have, I think I might prefer it overall just for the hub it’s become.
Webex Teams but that is not on the list! Started with Slack then to Webex Teams 3 years ago, combined Slack/WhatsApp & Video Conferencing into an Enterprise app.