As of September 2014 the smallest (15KB!), most minimalistic app for simply taking notes is called Zen Notes from Peter Gyorffy.
It doesn’t have any features like reminders, changing the theme or font or any other useless stuff. It’s basically like a sheet of paper where you can write your thoughts on, just digitally. The only thing it can do is share the written text over whatever Android offers you, e.g. email. When you exit the app the notes are saved automatically.