I use Photoshop Mobile. It crops, sharpens, softens and some other basic functions. Plus it's free. But like Alli said, it depends on what your needs are.
I would definitely go for Photoshop mobile first (is free). If you're not satisfied than try Colorsplash out (it should cost $0.99 if I don't get wrong).
I'll use PS mobile for basic editing, but if there is a particular effect that I'm looking for, I have a few apps that I use. Pano is good for creating panographic photos. cinemaFX has a lot of nice presets that you can add to your picture to give it different looks. Also, Tilt Shift Generator is great for creating tilt shift photos that give you that "miniature" look to your pictures.