Testing a lot of markdown editors to replace both of my favorite that turned bad with their rental business model, I would end to use Mweb, which offers a lot of features to work with.
1. A fully multimarkdown compliance, that make it really useful to write from blog post to social science papers ;
2. A complete range of display modes (3, 2 or 1 column, both markdown and preview mode, a folder manager and a file manager in 2 left sidebars, all that can be visible or not at your choice ;
3. A library mode, to store your posts in a database, and allows you to build a blog or website - not that easy to configure, though ;
4. A external mode (internal and external open different windows), that allows you to open any folder on your mac and manage the text files inside - combine with dropbox, you can share this folder with many apps on your mac and your other portables devices. Plus, Mweb can deal wtih subfolders, that helps to keep your files in order;
5. An integration of a range of publishing services, like Wordpress.org and .com, Blogger, Tumblr, Medium and metaweblog API, plus some photos sharing service like Google Photo and Imgur.
I first use the lite version of Mweb, asking myself what it misses, but when I bought the pro version, I really appreciate the enancement. A bar is added to the main window, making it handy to switch between display mode, add main markdown features (including a neat and smart table manager !!)
Of course, you can export to HTML, PDF, RTF and DOCX, and send directly to Mail, Twitter, Linkedin, Pocket, Notes, Messages and DayOne.
Impressive, isn't it?
In brief: I came to try nvALT to integer a markdown notebook, and to find some apps to replace Ulysses (really great but SoulMen became just to greedy), and DayOne (that I love, but not be sure of its future, as I only need DayOne Classic). Mweb made more: I begin to use it to replace both, but also nvALT and my main notetaker for years, MacJournal. It will be my notetaker, my blogger, my article writer on mac, and guess what: I can share all my files with the iOS app (universal), using the external mode on a dropbox folder.
This is simply the best macOS markdown editor I tried (and believe me, I tried a lot of them), but it's more than a simple markdown edifor. It's a complete writer suite with open markdown files.
Though I love it, it misses some cosmetic features, including an easy way to change and share themes (as Ulysse has), and a better images integration (a dedicated folder where do search when it's time to upload posts with images).
I don't really understand why this app is not on the top five of any markdown lovers.