Recommended

• iPad: Savage Interactive Procreate (iPad)
• Android tablet: AutoDesk Sketchbook Pro (Android tablet)
• Android phone: AutoDesk SketchBook Mobile (Android phone)
• iPhone/iPod Touch: AutoDesk SketchBook Mobile (iPhone, iPod Touch)

Honorable Mentions

• iPad: AutoDesk Sketchbook Pro for iPad
• iPad/iPhone/iPod Touch: Meritum Soft's Mertium Paint Pro (iOS)
• iPad: FiftyThree Paper (iPad)
• Android tablet: AutoDesk Sketchbook Express
• iPad: AutoDesk SketchBook Express for iPad
• Android phone: AutoDesk SketchBook MobileX
• iPhone/iPod Touch: AutoDesk SketchBook MobileX