Interesting. I suspect something like this will always be subjective, but I find the opposite to be true. *bold* and _italics_ make the most sense to me and is always what I wished Markdown did.
Probably, this is because I was familiar with Textile[1] before I used Markdown, and this is what it does.
Today, Slack also uses this convention instead of the Markdown convention (though I believe it _used_ to use the latter).