Zulip: The most complete team chat solution.

Feature	Zulip	Slack	Mattermost	Discord
Apps for every platform
Hundreds of integrations
Self-hosting available
Emoji reactions
Markdown formatting
Topic-based threading
Lightning-fast search
Free and open source			Open core *

* Mattermost Team is open source, but many features are only available in their proprietary version, Mattermost Enterprise.