Pythonista. Great Python editor, REPL, and platform-specific libraries.
Editorial. Same author, Markdown editor which can be scripted in Python. I routinely write al new features for it, like dice rollers, list renumbering, etc.
Documents by Readdle. PDF & epub reading, file management, bunch of optional features.
iCabMobile. Browser with a ton of ad-blocking, filters, good file management, I routinely use it as a private browser.