I tried many apps, but I truly fell in love with Textkraft! It's by far the best app for productive writing on the iPhone! Especially the built-in dictionaries and the cursor navigation tools make the app so outstanding! I also like the text information feature (only available for iPhone 6, I believe). You can get plenty of statistics of your writing with just a single tap: Textkraft displays you the number of used signs, words, paragraphs, pages etc., really helpful sometimes! There are 3 different color themes to choose from. My favorite is "night hawk", since it's the best theme when I'm taking notes before I go to sleep.