I recently settled on Appigo ToDo which I purchased for my iPad and iPhone (yes, a separate $4.99 purchase for each, but well worth it for the automatic synchronization across devices). I bought it mainly because it can create tasks that are linked to GoodReader files. I review or read for reference lots of files that I store in GoodReader. With ToDo I can set due dates and reminders and the links make it a lot easier to get to the file. However, I have found that I really like ToDo as a todo app and use it for lots of other business and personal todos.