My Desert Island App

We've all heard the familiar question before: You're stuck on a desert island. If you could bring one thing with you, what would it be--and why? In recent years, my answer to such a question was simple: my cane. Without it, I'd be in big trouble. If given more choices, I would invariably include things like food and water...and my iPhone. Oh, and speaking of that iPhone. If I had to choose only one app to have on my phone, what would I choose? The answer, for me at least, is not an easy one. My iPhone is about practicality. I don't do any gaming, and I use my phone mainly to make my everyday life easier: I text, read e-mail, check Facebook and Twitter, keep updated on the weather, read the news...that kind of thing. So, with that said...my "desert island" app would be Facebook, because I can keep in touch with all of my friends and family--not all of whom have Twitter. What about you? If you could have only one app on your iOS device, what would you pick? Let us know in the comments!