I mostly use Highland 2 - but even then I use the iA Writer font iA Writer Duospace which is exceptionally nice.

As regards Highland 2, in addition to its own format it will load standard Markdown files with a hierarchical document map. Native, fast, light on resources, and the file format is both documented and simple (basically Markdown in a container format).

Seconded, iA Writer does a great job of getting out of the way while having enough features to make it pleasant to edit text.