iOS: Pixelmator is the best desktop image editor out there, and the iOS versions are pretty great too. Today, the iOS apps get a much better Repair tool and a new Dynamic Touch tool that makes drawing a lot easier.
The Repair tool is a lot faster and easier to use, which is nice, but the big addition is the Dynamic touch tool. Gently use the tip of your finger and you'll create thin brush strokes, press hard and you'll make thicker lines. It's a simple thing, but it works really well.
Pixelmator ($1.29) [iTunes App Store]