I'm a part of a ton of Slack communities but I'm active in only a handful.
Slack works to its fullest only when it is implemented in a team and everyone's supposed to use it for communication. Slack could/should improve on making the communities on it better by adding relevant features.
As a remote designer, this is a must have. I'm always contactable and available. Super easy way to also have quick calls and communicate with other team members on different teams.
I have used it extensively in Andrew Chen's talk platform: http://talk.andrewchen.co/ and also for Write The Docs forum at http://forum.writethedocs.org/. I think it meets most of the goals of a well structured, well monitored, and custom discussion forum. I like it.
Telegram keeps on releasing amazing updates. Secure, easy to use messenger with great UX is all you'll ever need. I wish more people used it rather than Whatsapp or Messanger subpar experience.
Favorite messenger, has its all: super fast, great UI and UX, chats foe different communities with quality content, bot platform that is growing rapidly. New releases coming frequently.
The biggest wish is to get rid of notifications that your contacts are on the platform (IOS)