Muzy has one of the most robust set of features we've yet seen in a photo editing app. You won't even have to put up with ads to use it, either.
This app makes you log in before you can start editing your photos. You can save some time by connecting your Facebook profile. If you go that route, you can edit all of your Facebook photos and even some of your friend's photos. The app doesn't stick to one type of photo editing; it lets you create collages, add filters, crop, retouch, and more. There are actually so many different features that you often have to scroll through the menu multiple times to find the feature you want. That's the only knock on the app's interface, though. Sadly, you do need to install add-on apps to get to some of the more advanced features. There aren't any ads, but the app does send the occasional pop-up notification at you.
If you want one photo editing app to rule them all, Muzy is definitely it. It lets you do so many different edits to your photos and lets you do them all very, very well.