I like Byword for writing. It has a very simple interface that I've come to associate with writing. When I want to just sit down and let my fingers put the words in, I find myself drawn to this application.
I've tried others but I feel my brain is now weird to think that writing = byword.
The only problem is when I'm out and about and I just have my phone. I don't really have a comfortable way for me to put something like this blog post together without my laptop. I'll have to come up with something.