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I don't know about the best but check out Hipstamatic, Camera+ and IncrediBooth. All excellent.
 
Also interested. I'll be specific. An app that does basic adjustment functions, such as crop, lighten - darken, sharpen etc.
 
25 camera apps later, I find I prefer lo-mob (over hipsta), Camera+ and PS Mobile. Diptic is also neat but not one I use all that much.
 
I have both lo-mob and Hipstamatic. I like them both, but in many ways I prefer lo-mob for its simplicity. But they are both fun in different ways.
 
I used to use iFlashReady for lightening photos on my 3GS; it does a far better job than PS Mobile; however, it had a file saving bug on my old iPhone 4.

regardless of how many filter apps I have my go-to phot apps are

1. PS Mobile
2. Shake-it Photo
3. Lo-Mob
4. Camera Bag
 
TiltShiftGen is great for adding depth of field.
Photogene I use for adjusting levels.
PS Mobile will handle anything else.
 
At the moment, I'm heavily using:

Diptic
Camera+
TiltShift Generator
PicTone
Camera Kit
 
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