I hate reviews that make you click through a bunch of options before getting to the point. So please allow me to tell you here what I think the best system is.
What? Nothing? No, not nothing. It's a combination of apps that make up a very helpful system.
These 4 things work together to offer what I'm looking for, both with capabilities and without lockin. Here's how it works:
As a Mac user, it's available everywhere. If I need it on a Windows machine, I can install iCloud.
Most importantly, I scratched out Dropbox because without notice, they instituted a 3-device limit to their free tier. As someone who was a casual user, that's fine; they have a right to stay in business. I didn't care for how they did it. I've used the business level of Dropbox before, and I was pretty happy with the "just works" aspect of things. However for a casual user, you'll run into this limitation fairly quickly... maybe. If you don't mind 3 devices, it's still a very good service. However since I'm paying for iCloud, I'll be sticking with them.
Why is a format part of the solution? John Gruber's original approach, and the one adopted and transformed into what's known as "GFM", is a way to write plain text that is formatted. This means hashtags for headings and asterisks for bolding. It is completely readable as-is. It's not as pretty, but it is obvious when you read a completely plain text file that what you put underscores around is intended to be emphasised. It's a little like "shorthand html", but instead of writing
<h1>Heading</h1> and surrounding all your text with HTML, you just write
# Heading. This means that not only do you not export your files, you simply work with the raw file as it is. Using certain smart applications will apply those formats, and that lets you keep your original in an "application-agnostic" format.
With that said, MultiMarkdown is awfully handy with its include capabilities. Code fencing is still there, but you give up some auto-linking and task tick boxes.
Typora is my main writing app on the desktop. It does one trick that no other app (as of yet) does, and it's amazing. Unlike other apps that have a "preview mode", or some that do side-by-side rendering, Typora formats GFM in real-time. This means as soon as you complete a formatting step, it's going to put it on screen within the rest of your writing. Personally, this makes writing in Markdown much, much easier mentally. Instead of switching into a mode to see how things witll work, you see it immediately.
For iOS, 1Writer is excellent. As a mobile editor goes, it is far and away the easiest and cleanest. Easy Markdown previewing, uses GFM, code pigmentation, and auto-naming.
Using these 4 keys in combination, it allows you to do all those tricks, plus a few more that I mentioned before:
If you use any type of code, or want to call out certain text clearly and in a monospaced font, Markdown allows for a "code fence". Start and end with three backticks. It's the ~ key without the shift, and you get a fully different experience.
$ This is fenced code.
If you write anything this way, you'll understand exactly why you need it. Try it out.
Part of the GFM upgrade for Markdown is that it allows checkboxes:
The ease at which content can transfer is also very helpful. Whether it's notes, or in GitHub or Azure DevOps (Fire the marketing team who named it that), or in a system like this website using Grav, Markdown content can be transferred in and out with all of its formatting fully intact.
Hopefully this helps, and lets you avoid note lock-in!