My go-to Dropbox-powered, Markdown-equipped iOS text these days is Notesy. The $4.99 universal app is fast, powerful and has been updated for iOS 7.
Before using Notesy, I was a die-hard PlainText user. PlainText was one of the first apps in its category, worked well and looked good.
Before iOS 7, the app was free with a single in-app purchase to remove banner ads. While I was no fan of this method, I enjoyed the app and paid without any hesitation.