Asciidoc seems to have a solid backend. But it seems to have a problem with nesting. Nesting things should be tablestakes in a markup language.
I don’t know much about RestructuredText (no, I won’t play these silly free-caps games) but it doesn’t look that nice to me for whatever reason. It looks the closest to regular markup to me except they have replaced things like brackets with backticks.
And finally I am glad that I never have to use some Wiki lightweight markup variant with silly syntax like using X numbers of apostrophes for emphasis or whatever.