The one built into confluence because then someone else can edit it. But I am not an architect. If I was I may get a dedicated tool.
I'm guessing for a lot of people draw.io and Excalidraw are probably the go-to. If you use draw.io (or something else), what do you like about it, or what do you wish was better?
[1] - https://app.vexlio.com/ for the curious