Slack trains machine-learning models on user messages, files and other content without explicit permission. The training is opt-out, meaning your private data will be leeched by default.
I can’t really tell you which one is the best, since I never used any of these (except for Session) for an extended period of time. Briar seems to be the best for anonymity, because it routes everything through the Tor network. SimpleX allows you to host your own node, which is pretty cool.
Stay away from proprietary crap like Discord, Slack, WhatsApp and Facebook Messenger. There are enough FOSS alternatives out there: