I need to Process Markdown at the command line, so there is no real language restriction.
Extra features such as pretty quotes and dashes are a welcome addition. Code blocks are a must, but they are part of the core I think.
I want to output to HTML only, and I'm on a Unix environment.
I'm not sure what the most maintained is, and I'm certainly biased by using it here most, but the one SO uses is also available on github, it's the markdownsharp project, written in C# and pretty well maintained.
It fixes many of the bugs in the original markdown implementation, found by SO users as questions were asked, like most things, just a project that came out of a necessity. That being said, some of how markdown works is as designed per the spec, but isn't necessarily intuitive, it's the maintainers prerogative there as to what action to take there, intuitive vs spec...so you may have to adjust any implementation a bit to get exactly what you want in some edge cases.
I've been using python with http://www.freewisdom.org/projects/python-markdown/ without any problem so far, and it has a simple extension mechanism.