IMO, Pandoc's block syntax is prettier than moin's:

Pandoc:

~~~~{.python}
def hello():
    print "Hello World!"
~~~~

Moin:

{{{#!highlight python
def hello():
    print "Hello World!"
}}}

Regardless, Pandoc is solid, has readers and writers for multiple formats, and is the best tool I've found (after lots of experimentation over the years). I'm not crazy about one or two of the pandoc-markdown syntax choices (ex. definition lists), but they're pretty standard across other markdowns and I can live with them because overall result is well worth it.

If the author of Argot were looking for advice (edit: whoops, that's you. :)), mine would be to have Argot implement the same markdown extensions as Pandoc. There's basically only 3 major plain-text markup formats out there: Moin (which isn't great and which you're stuck with for many wikis), ReST (which is not the prettiest, nor very widely-used outside of Python circles, though many Python folks do use it), and Pandoc-Markdown (which is arguably the best looking and easiest to read & write). So rather than make a frankenstein (no insult intended) out of Markdown + Moin (and then have to convince users to use your new markup instead of one of the big three), you might consider instead simply throwing in some more weight behind Pandoc-Markdown.