Zoom meetings, definitively most valuable, but Zoom webinars are also quite good. 

Slack can be used right away also for small companies. I personally do not need tons of integrations. It is beneficial on its own. And it runs super stable. 

My primary use case for Slack is chat. Private chat, group chat, chat in channels, etc. This includes file sharing or pasting screenshots in the chat is quite helpful if you want to explain something. Sometimes I also go on a huddle, but that's not the primary use case. 

Channels and channel search is what I use most, in addition to common workspaces with partners.  Especially those channels where everyone can post can be so beneficial.  It is so easy like a discussion forum. You can search if someone had a similar question and if you cannot…