Typora for me is the best application for writing Markdown on MacOS. Many applications do a great job of text editing and previewing, but only Typora does real-time preview. It will change the way you think about markdown.
For iOS, it's the best application I've found for working with markdown files. It will sync with iCloud and Dropbox, and it will preview markdown using GFM.
FSNotes is the closest app to a winner for me. It handles files directly as .md, so you can select a folder and directly edit markdown. There is a matching iOS app as well. All of this is good, and it's even free if you can
brew. However there are a few shortcomings for me personally... the real-time editing of Typora is better, and the iOS app can't handle Dropbox. Those shortcomings might even be advantages to you, so check it out.
A fantastic full ecosystem within the Apple world. It integrates seemlessly, syncs well, and has a lot of great features. Where this doesn't work for me is two areas:
What was one of the most promising applications, I've become very sour with this app. I had data loss, and that's just not acceptable. It's one guy working part-time on the app, and the promised iOS app never appeared beyond a read-only. Some really neat tricks in the app, as you can combine different kinds of note pieces, such as text, code, etc. Really, it's nothing you can't do in markdown, but the conversions to a presentation mode are slick. If you're on one Mac all the time, it's worth a look. Just make sure you make backups.
A great free service, you're limited to two devices when free. It does a lot of note-y things, but doesn't handle code well, doesn't export well, and is a subscription service. Not interested in anything where I pay monthly or lose my stuff.
It's a json-type app, and it costs per month. Very clean, but lacks features in a way they call "minimalistic". Whatever.
MWeb is a very interesting application in that it tries to do a lot of things well, and gets some of them right. It offers iOS with full sync and it keeps files in an .md format. Unfortunately it has a "library" that is all self-titling through a database, and as such allows for some organisation but sacrifices files being in your normal structure. There's a secondary way to look at a folder that's good, but it's no better than Typora opening a directory. It can also publish to websites and blogs. Not bad, but I'm happy with Grav because I can publish directly to GitHub and it auto-syncs.