Well I make https://gojs.net, so I just use the GoJS diagramming library to make diagrams :D
Of course, its made for developers trying to make applications, not end users.
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
Of course, its made for developers trying to make applications, not end users.