What are the best Markdown editors for OS X?

Markdown is a plain text formatting syntax for web writers that is easy to read, easy to write and designed to be eventually converted to rich text while remaining independent from any application.
Things to look for when choosing a Markdown editor include how it makes writing Markdown easier, how it previews rendered Markdown, how much customizability and options both in term of the editor as well as output are available and if it supports a Markdown flavor you like.