Typora is the markdown editor I love. It’s the one electron app I use, because it has one edge over any other markdown editor I played with: a seamless live “preview”. The markdown syntax and the rendered version blend into one another beautifully, so you don’t have to toggle between them, or go with a side-by-side view. That’s the killer feature for me.
It also allows for user-defined rendering styles using CSS and provides a sidebar with the document outline or a file tree, which I like as well.
It’s currently in beta and usable for free, but probably will switch to some payment model once the “1.0” will be released. It’s been a beta for quite a while now though.