For basic color correction and exposure adjustment, it's hard to beat Snapseed. It's easy and intuitive to use, has lots of useful features, gives great results, and is free. The iPad's camera is great when it's not too challenged by lighting conditions, but when you get on the wrong side of its limitations, Snapseed can often salvage the photo, sometimes even turn it from dismal to spectacular.
To take pictures, I recommend using Camera Awesome! Instead of the standard iPad app. It's just better in every way.
I've tried and deleted dozens of photo apps, and these two are still on my iPad. Two other useful ones are Phonto and Skitch, for adding type and arrows to images.
milliHelen: amount of beauty required to launch one ship.