Pretty tough question as there are so many.
If you are looking for picture taking apps with a twist, I like apps like 360 Panaorama, CamWow is pretty versatile and silly – multiple lenses and effects to take photos with that do things like mirror, pixilate and distort like a fun house mirror. For Post Production, I use Photoshop Express to sharpen, crop, straighten, and change exposure. Diptic to put multiple pictures in one large frame and Pic Collage to make, well, a photo collage. There are a myriad of other cool apps I have, but again it depends on what you would like to do.
Hope this helps -