What is the #1 photography app for iPhone?

I plan on having my own photography business and I'm wondering what the best app there's to use. I want the app to include action shot/ stopped motion. Another question, is it best to get Photoshop on your iPhone or actually use a computer?

Most likely Instagram considering it's popularity, if you can call it a photography app. Besides, if you're planning to do serious photography then using a computer is your best bet.