What is the best free photo editing app for the iPhone/ iPad/Android?

The best free photo editing app for the iPhone/ iPad/ Android is, in my opinion, Lightroom. It is free and contains nearly all of the editing tools that you need. You can take photos in RAW using the built-in camera, organise, rate and edit photos, and share or export photos straight from the app. And if you pay to subscribe (easy for me to say) to the Creative Cloud Photography plan, you can sync photos with Lightroom on the desktop. And that's what I do.