I really hate that Teams has basically taken over. It has so many poor design decisions that I regularly feel like I am fighting it.

My single biggest pet peeve, how it handles chats from meetings when you are not in the meeting. I have lost so many chats (or never got responses) because they were hidden in meeting chats that for some reason don't appear in your normal chat list.

Sure Slack had its own issues, but I don't remember ever feeling like I was actively fighting it and loosing.