In our projects, we write our spec files in Markdown - most platform-agnostic way to do it, as everyone uses different types of computers. Having a good Markdown editor was essential for me, and I spent a lot of time doing research and testing several of them out - on the Mac.
IMHO, the best Markdown editor - which you probably never heard of - is the amazing LightPaper. It has loads of useful features that make it stand out from the rest.
As far as Mac editors and IDEs go, I've tried most of them: BBEdit, TextMate, Sublime Text, Atom, Espresso, Smultron, SubEthaEdit, Coda, and Brackets. They all have drawbacks, and they all have their strengths. Sublime, for instance, is by far the fastest, and most extensible, but the interface is truly horrendous: all configuration has to be done via a myriad of text files, using raw JSON. Expect to spend months adding and fine-tuning plugins, to get it to work exactly the way you want it - and to learn all of its keyboard shortcuts. When you eventually do, it's a powerhouse. Coda, on the other hand, is fantastic for the beginner: beautiful interface, super easy customisation, loads of bundled tools - even the complete set of documentation on HTML/CSS/JS/PHP is built right into the program. But it does not have as many plugins as the others, and power users often end up moving on to more customisable editors.
Most editors have plugins that allow you to do some processing, using languages like LESS, Jade, Coffeescript, etc. Some editors can compile your LESS files, for instance, as you save them. Others might even allow you to run autoprefixer over the compiled file. But none of them does everything - such as easily compiling, post-processing, concatenating, minifying and previewing, all together - not even with plugins.
In order to get a completely automated workflow like that, you will usually need to use command-line tools: Bower (for managing dependencies on frameworks and libraries), Grunt and Gulp (for automating pre+post-processing tasks), with add-ons like Browsersync, to be able to preview your changes live, as you go. But each one of these tools requires time to learn, time to setup, and end up greatly increasing the complexity of even the simplest projects. They also require you to clog up your computer with dozens of command-line apps, extensions and add-ons.
And that is when I came across CodeKit. This is a very clever program, that bundles all of those tools in a neat, graphical user interface, that is unbelievably easy to use. It is now by far the most important tool in my programming toolbox - and the best $35 I ever spent. It allows me to use whatever editor I please, giving me the full power of Bower and Gulp, as well as setting up an automatic previewing server, without me having to instal any command-line tools in my system, or any plug-ins in my editor. It makes using these fantastic tools amazingly easy: no complex setups needed, it all just works - and it freed me from worrying about editors...
I hope this information helps.