iOS: You have a billion options for different photo-editing apps on iOS, but Filters might be one of the simplest to use. It comes packed with over 800 different photo filters, but it still manages to be friendly to use.
Filters divides up its photo editing options into different categories and from there you can manipulate photos to your liking. Even though it has a lot of options, it's still easy to pop in, edit a photo and get out quickly. You can also add layers with light leaks, overlays or textures. If you're looking for a single filter app to rule them all, Filters does the job.
Filters ($1.29) [iTunes App Store]