....is the one you have with you!
I can't recall which photographer said that, but never has it been more true. I use an Android smart phone, and over the last 12 months have tried pretty much all of popular the photo editing apps. At the moment I'm hooked on 'Magic Hour For Me'. I'm impressed just how much control it gives you overthe image; colour management, textures, borders and vingnetting (an almost infinite combination). However one of its main strengths is the ability to download users filters uploaded to the Magic Hour market place (all of which are free!!). As you can imagine you are limited somewhat by the resolution and quality of your phone, however, if like me you see photo opportunities everywhere and don't always have your camera, a quick shot can be transformed into something pretty cool and uploaded in seconds. Check out the Flickr group dedicated to the app.