Presentation editor: Keynote
Simply put, Keynote is an amazing slideshow editor. I prefer it over PowerPoint even on the Mac, and on the iPad, it works beautifully when creating complex slide transitions and element effects. And its iCloud compatibility can be a real life-saver, ensuring all your devices have the current documents. A bonus is Apple's free Keynote Remote app for the iPhone and iPod Touch that lets you control a Keynote presentation remotely on your Mac or iPad.
Keynote's big negative is its awkward requirement of copying documents to and from cloud storage services, rather than allowing direct access as other apps do.