The main thing you need to know Slack is their slogan: "It's the foundation for teamwork." Right from the get-go, they declare themselves as a platform for cooperation—no shenanigans about family or connecting with people you love—only business, only hardcore. As a result, you get a solid product with a neat UI made for managing multiple teams; apart from separate groups for each project, there can be spaces for the executives or for all the employees, or even, let's say, a room for rock music fans.
You may think it sounds the same as previous picks, but there's something to it—the UI is simply well-thought. The comment section is designed to avoid clumping the messages via sub-comments for each of them. Furthermore, you can generate a link for each message, share it with others, start a thread, and add reactions that can be uploaded and customized within your own corporate network. File sharing is also what deserves praise thanks to Slack being a Cloud app, making it simple and safe to exchange all sorts of documents, complemented by a detailed preview feature.