Slack is the industry standard for managed instant messaging (IM). A good alternative would be to self (or cloud) host an open source IM such as Mattermost but as always it would be a good idea to do a cost benefit analysis between the solutions.
Some of the main things to consider:
- Having a good SDK for plugin creation
- Having good integrations with existing tools ( JIRA , GitHub , OpsGenie , etc.)
- Maintenance and administration
- Covers all your businesses use cases