Best functionality for depth of use cases. Remote teams, synchronous work (e.g. video chat) asynchronous work (to dos, scheduling, backlogs, etc.) AND semi-synchronous work (chat, messaging etc.) with private and public channels (so you can do better brainstorming etc.).
Also love that I can easily create channels for external guests. E.g. I have a single channel with customers in them so we can chat and I can stay connected with them and I have another channel where my PPC agency can collaborate in real time with the rest of my marketing team. I can create a highly agile and inclusive collaboration experience with anyone who matters, with any context, in real time but without exposing any internal communications that are either private or unnecessary to share.