I love Snapseed and consider it as the best mobile photo editor with a bunch of pro features to make any photo better. I use Tune Image and Curves the most. I especially appreciate Expand function: I use to add some background before rotating a picture to align the horizon — it helps not to cut important details.
What's better than a app used by most Instagram photographers? This app is one of the few things to come out of Google and still be loved by its users after years.