What I use to write and maintain this website:
(I finally(!) got a Retina iMac. More on it later...)
I also have a semi-retired 2010 27" iMac which currently functions as a server for files, Time Machine backups, and Xcode builds.
On the go, I use my 128gb iPhone 6S. On the couch, I have an original iPad Air 64gb from 2013.
I capture most of my notes, thoughts, and initial ideas on Drafts for iPhone. I sit down to write the text of my blog posts and web pages on Scrivener. (I'm currently trying out Ulysses to see if that works better for me.) For markup, I use Markdown.
These other apps also come in handy quite often: