Occasionally it can be hard to wrestle with but my go-to is PlantUML, which has a simple syntax and for better or for worse doesn't support many formatting options, so I can't get distracted tweaking the specifics of the layout
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