It depends on the use. If it is simple enough, mermaid diagrams have several advantages, like easy to understand versioning, and integration with many note taking programs.
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