Mattermost is the most obvious option; it's a clone of Slack. IRC is another good option, although I know a lot of people hate it because they prefer features to freedom. I cannot recommend Matrix; the UX is fine but the cryptography has a few issues, as documented by Soatok here.

Revolt chat. Pretty much an open source discord.

IRC, XMPP, Matrix (in order of personal preference)

Matrix and matter most are my top two. Matrix is preferred because of the federation support and a pretty good bridge (to services such as discord) ecosystem.