In our case we still use Skype. It's the only native cross platform app that offers group calls and screen share for free.

I'd gladly use Slack, but group calls and screen share is a paid add on.

Hangouts has always had terrible performance in my experience.

Facetime works nice for one to one, but a lot of people don't have Macs, and again no group calls or screen share.

I liked Fuze, but then they pivoted to enterprise paid clients only.

Bluejeans is paid only and via Facebook IIRC.

What else is there?