I have a ton of photo editing apps, but a few favorites are:
General photo editing w/ cropping & straightening:
Snapseed
Iris
Photoshop Express
Photoforge
Photogene 2
Luminance
Image processing w/ no cropping or straightening:
Process (I really like the way this app handles adjustments as layers. It's very non-linear, and easy re-edit each adjustment, also allows you to save "processes" to the cloud for future use, and to share them by email. I can't recommend this app enough. Get it )
BTW, I am not affiliated with the Process app at all. I got it recently, and it immediately became one of my favorites. It might be my favorite photo app, but lack of cropping and straightening limit it somewhat. It is pretty awesome for what it does, though.