Best value for money online team collaboration tool?

Hi, I'm looking on the market for online team collab. tools. New gen. ones, multiple platforms support (android/ios/pc), web based, secure, integration with remote storage (e.g. dropbox and the like), searchable, meetings/calls and the like. But -as usually- my (medium size S/W house) company is very much concerned with maximizing the value for money factor but on the other hand we want to go with a known company. So long I'm peaking into:

1. Slack (the market leader AFAIK)

2. Google business hangouts

3. IBM Watson Workspace

4. Microsoft Teams

Are you using any of them? Happy? Not happy? Any reviews/commends/pointers welcome.

Thanks