Totally depends on how powerful/complex you need the app to be. Are you looking for simple color adjustments, or the ability to alter curves and levels? Do you need to be able to add text to your photos? Do you just want quick and dirty Instagram-style filters? And so on...there are SO many apps out there that the answer really changes depending on user needs.
I'm a heavy photo editor, for what it's worth, and I mostly use TiltShift Generator, Photogene2 and Photoforge 2, AutoStitch (for panoramas), and Photoshop Express for some quick fixes. One app I really love for creating oddball photos is ToonPaint, which does a pretty incredible job of turning photos into line art.