Excalidraw and PlantUML
Each one has its own benefits / drawbacks.
PlantUML is great for version-controlled files.
Excalidraw for throwaway diagrams and discussion. Ocassionally I draw comic strips with Excalidraw.
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