Chat + Forums + Fileshare + Video chat + Voice chat + OneDrive integration + Sharepoint integration + Kanban + goodness knows what else
Before the lock-down we all turned our nose up at it as "yet another lame chat app", now it's dominating everything, and it works on MacOS, Windows, iPhone, and Android -- so there's no excuse for anybody to not have it on their phone as well.
There's nothing else that's even close for us and just on my team we have about 60 people on it, and across the company over a thousand. It's almost 100% replaced all the other things that do the things in the list above with the exception of Jira and a few holdouts still using Webex (so there's a dial-in for people outside our org who don't have teams, but we've even been using for meetings across different organizations that have teams and it works perfectly fine).
Duo, Meet, Hangouts all seem really spare and kind of old fashioned now.