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
Graphviz - same basic reason as Mermaid, though no markdown support. Versioning text is a lot easier than versioning binaries.
Draw.io - if you've got to have a binary, this is it. In particular, it allows you to embed the drawing information in the image so that you can import a .png file into draw.io and get the drawing.
And if I need to generate a graph programmaticaly then I instinctively reach for Graphviz as it's solid and can produce the graphs in so many different file formats that they're easy to include wherever they're needed. Your code is a lot simpler as it doesn't need to handle any of the rendering logic, it just needs to work out which nodes are connected by which edges.
Regarding graphviz/dot, I have been using it from what feels like the dawn of time and am somewhat sad that we haven’t improved upon it. The syntax, styling imitations, etc. put me off enough that I tend to avoid it.
For graphviz, the "problem" with it is that it truly is general purpose and trying to make a general purpose syntax for diagrams is difficult. Mermaid tackles this by not having it be applicable to all possible graphs (flow chart has a different syntax than gitGraph) allowing for a better domain specific language for each subset of functionality.
https://mermaid.js.org/syntax/packet.html is really neat... and it would be a pain to do that in GraphViz ... but I believe that's more of a demonstration that it might be better to do specific syntaxes for specific types of diagrams.
The most general Mermaid form is https://mermaid.js.org/syntax/block.html - and that syntax doesn't really feel like it is an improvement over GraphViz.