Snapseed's brush tool is by far my favorite photo editing tool.
This one tool will make you a professional photo-editor in minutes.
Before beginning, here are a few tips for using the brush tool:
Pinch the screen to change brush size
Tap square icon at the top right corner to toggle on/off the edits
Each brush preset has 5 settings based on strength: -10, -5, ERASER, +5, +10 (exposure has 2 extra)
Saturation - make colors brighter or darker
Temperature - alter colors slightly
Exposure - increase or decrease brightness
Dodge & Burn - increase or decrease shadow depth
Eraser - Undo parts of an edit without undoing the entire edit