Slack is the worst of all the options. As the user before me mentioned Discord and Riot are good options. But the best option has to be Telegram as you are required to have a # to get involved there. That means no bots and multiple accounts, only real people.
Thanks for your answer! The big issue I have with Telegram is a) user behaviour b) lack of overview.
Due to the single channel messaging, messages get lost very quickly. IMO this impacts user behaviour as well and lowers the quality of the posts and discussions. Probably it's key to define what Telegram is for and what not - and channel quality discussions somewhere else right away.
Btw. technically speaking, what does the # stand for - how can it prevent bots?