There's SnapSpeed which is pretty user friendly and very powerful. There's also Adobe Photoshop Express, which it a little more basic but still has a bit of a curve. Really depends on what you're looking for.
On the flip side, Photoshop Express can edit multiple RAW formats from regular cameras, while Snapseed can only edit .DNG's that a phone creates. It also has a few extra features like noise reduction control. I also like this one for having a great text editor.