Sublime Text for code, Mou for editing Markdown, iA Writer when I need to write long form blog posts. I also occasionally use Google Docs & Pages when I need to edit documents for college.

Mou, to the best of my knowledge, provides the best real-time live preview of Markdown code. iA Writer really helps when you need to focus (had tried Byword previously but didn't impress me -- maybe I'll give it a try again.) And, well, what can one say about Sublime? Love it.