Using $ and $$ is most legible for me. Having spent many years using TeX. Pandoc seems to do the same thing - I suspect for the same reasons. I usually patch this into markdown systems that I use. This is especially useful when you have have to talk about things like $x$ and $y$ and how $x^2+y^2=z^2$. In actual math documents, every third or forth word will be surrounded by $, it is almost like italics to mathematicians, and would be nice to be able to express it just as easily as italics.

I find it a lot more readable than the \( and \[ that LaTeX and other markdown systems use.

I believe gitbook uses just "$$", but this looses the distinction between inline and display math. (And there is a subtle difference between display math within a paragraph and display math between paragraphs, so you can't tell from context.)

I understand that some people will want to talk about money and having to escape $ could be incredibly annoying, but for actually writing math documents, this is kinda essential. So, to me, it feels like something that needs to be an optional add-on.