Bear (http://www.bear-writer.com) supports Markdown and has cross-device sync via iCloud. It's not the best for writing code but if you're copy/pasting existing code from another editor it's good.
Bear is great for organising notes. The style of #projects/open-source/react for example makes it really easy to be as broad or direct as necessary. Makes finding notes where you're not entirely familiar with the content much easier.
