Slack, by a wide margin. All internal communication runs through it, and because everything sits in threads, a decision from last year is still findable. The part that matters most, though: we’ve connected our systems to it, so exceptions arrive where people already are. Nobody logs into a dashboard to check whether something went wrong — they find out afterwards. Push the exception into the conversation and it gets handled the same day. That’s what makes it our hub rather than just our chat: it’s the one place where both the people and the systems speak up.