From a user perspective I really like Flarum https://flarum.org/

Some example forums that use flarum:

Flarum itself:

https://discuss.flarum.org/

GrapheneOS:

https://discuss.grapheneos.org/

Kagi and Orion:

https://kagifeedback.org/

https://orionfeedback.org/

Mailcow:

https://community.mailcow.email/

Many more can be found here:

https://builtwithflarum.com/

Discourse is popular and pretty good. I think Zulip could be better, but less popular.