Code markup in Apple Notes app

Notes in macOS High Sierra includes a Monospaced format.

In addition to the existing format types such as Title, Heading and Body, there is a new format named ‘Monospaced’. This format is a variation of the Body format but with a monospaced font family.

macOS High Sierra Hidden Features, Notes.app

Note: I am in no way connected to any of these software.

Free Options

Boostnote is an open source project for writing notes with syntax highlighting. This is a good free alternative to using the native Notes app if you aren't against downloading an additional application.

Boost Application

Paid Options

Quiver provides features like LaTeX support, syntax highlighting, customizable themes, cloud syncing, and version control. As of writing this, it costs $9.99.

Quiver app

Bear provides syntax highlighting, easy exports (to PDF, .docx, HTML, JPG, etc.), multiple themes, and syncing through iCloud. As of writing this, it costs $1.49 per month (if paid monthly) or $14.99 per year (if paid annually). If paying monthly, you will get a free one week trial. If paying yearly, you will get a free one month trial.

Bear app

Joplin

I've been using Joplin for a month. And it meets my requirements (I'm a developer).

Markdown editor [WYSIWYG]
Pasting image from clipboard
Dark mode (Also custom css support is there)
Export as PDF, Html etc
Dropbox sync
Tagging the documents

And yes, The UI is not intuitive, but the exported doc does look good.

Update [Sep, 2020]

I tried Obsidian and Typora.

Obsidian

Prefer if you're doing research work.

Some cool features

Easy Document linking and ability to update display text at ease
Keeps files & attachment in a so-called vault
Mind map to visualize how documents interact with each other
Presentation [Enable it in plugin section]
Theme support
Document Tagging

Cons:

WYSIWYG experience is not good
Image handling resizing not possible and copying the image also troublesome

Typora - current favorite

I think, I'll settle here. Unlike the other apps above this feels native. The other apps are built over (Chrome?!) electron framework?

Pros

Awesome WYSIWYG experience
Themes are looking good [It will get closer to Bear's UI]
Smoother experience with inserting/resizing & copying the image

Cons

Document handling is like file/folder open. Keep a separate folder for docs and set the same as startup folder in preference.