draw.io mostly, export as PNG and include the actual diagram XML within the PNG so it can be opened and edited.
very rarely use mermaid, but good for some simple README/markdown style diagrams.
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