After trying nearly every markdown editor out there (Mou, Byword, Macdown, iA Writer, and Caret, to name a few), I found Typora. I've been using it for the past year and find it delightful - it does everything that I want a markdown editor to do, in an elegant and intuitive way. I'm not tied to the company in any way, just really love the software (and hope it succeeds). I'll happily pay to support it when it comes out of beta.