Ask HN: Best chat software for an online university class?

College professor here. Owing to the pandemic, I've just pulled the plug on a class I'm teaching in elementary number theory, and moved it to online delivery.

I am hoping to get some lively interaction going on between classes, and I intend to use some sort of chat software for this. Officially, my university uses Blackboard for this, but as the currently #1-ranked entry in HN's "best comments" list explains, "the user interface is designed as if the programmers were incentivized to maximize the number of clicks required to get anywhere" and "it has the responsiveness of continental drift". No wonder my students barely used it when I set up a chatroom last Spring.

I'll therefore be looking for something else: Slack, Zulip, Discord are options I've heard of and am at least vaguely familiar with, and I'm also open to trying something new.

Any recommendations for what software to choose and how to use it effectively?