One gets jaded after years of looking at iOS image editors that are endless variations on a theme, or that have most of the tools you want but lack others. However, every so often an editing tool comes along that is well thought-out, a good performer, and free.
That brings me to DarkLight. This free app will likely become my default editor for a lot of my iPhone photos. It starts with Auto Enhance, a one-click way to quickly make a photo better. There are three levels of Auto Enhance, which is far better than the usual option found in the Apple Photos app to either enhance an image or not.
"Scenes" is another very nice feature. There are 13 different options to apply different enhancement parameters to a photo depending on where and when it was taken. There are scenes for cloudy days, landscapes, even photos of food. They all work well, and none goes so far as to wreck the original picture with too much enhancement.
Of course there are the usual saturation, vignette and tint tools, but we also get curves -- something you usually don't see in iOS photo apps and that can make a big difference to the look of your final photo. The app also has frames, stickers, text tools, and a tilt-shift option to selectively blur parts of your photos.
I think the only tool I found missing was a High Dynamic Range (HDR) type tool like the excellent one provided in Snapseed.
DarkLight is a really first class editing tool. It's free, and there are no aggravating in-app purchases to make. I also appreciate it contains a sample photo that lets you adjust settings to see how the various effects interact. I've added DarkLight to my main photo editing folder, and I think you should too. Author Harry Lindmark tells me there is a big update imminent, with some redesign and a few small bug fixes, but I found the current app stable and very user friendly.
DarkLight is a universal app (another bonus) and it requires iOS 6 or later. It is optimized for the iPhone 5.