> which is best?

Best is what YOU think is best. TMTOWTDI and no one can tell what's the best way if you don't define "best" ;-)
Is best the code that is

• fastest
• shortest
• least memory consuming
• clearly to understand
• at most obfuscated
• ...

Having said that, I would do it on a line-by-line basis, something like this

  while (<>) {
    # replace italics and bold
    s{^([ib])\s+(.*)}[<$1>$2</$1>];

    # find ordered lists
    if (my $hit= /^\[\s*$/ .. /^\]\s*$/) {
      if ($hit==1)      { print "<ol>\n";  next; }
      if ($hit=~ /e0/i) { print "</ol>\n"; next; }
      s[^][<li>];
      s[$][</li>];
    }
  }
[download]

This won't work with cascaded, ordered lists, but it's a starting point.

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