Everyone talks about how great apps are on mobile devices like iPhone and iPad. It's true, those apps are pretty amazing. That said, I think one app that deserves special recognition as being an amazing app, on any platform, is Apple Keynote.
I love Keynote more than any other app I've ever used. I use it almost daily. It's great for presentations, vector based image creation, basic image editing, collecting thoughts, telling stories, doing UI designs, and just about anything else you can think of. It's incredibly versatile and if you know what you're doing, an amazing productivity tool.
If you don't use it, learn it. If you're a marketer, designer, or creative thinker — there's no question that it's the best app out there.