Usually when I'm doing some writing, whether it be for formal publication or just reminders and notes to myself I'm writing in Markdown. It's easy to read, permanent, and can be turned into other useful formats like PDF or RTF.
When I first got into Markdown, I was using the old reliable TextMate along with a number of scripts and triggers. TextMate has gotten some real competition in recent years and has really struggled to keep up with editors like Sublime Text or Atom. I've spent a fair amount of time working in both of those editors and they're really something to consider, especially if you're used to using older editors like UltraEdit or TextPad which still see pretty wide distribution in Businesstown.
But even as I basked in the power provided by Sublime's column editing and the ease of use from Vintage Mode, Sublime lacked some of the Markdown editing chops that I really liked in Textmate like footnote support. Recently I've migrated from using Sublime as my primary editor to using Atom. The add-ons for Atom have matured quite nicely and with the right combination of tools and some configurations tweaks, Atom might be the best Markdown editor out there right now.
The other fairly interesting editor that folks seem to like is Bear. I like bear. It's super friendly and it's a great spot to keep notes that you need on a regular basis. I do wish it would sync out to plain text files though.