That sort of depends on what effect I’m going for! I’m problematic in that I don’t really have a set style that I adore, so I kind of just… wander around aimlessly at any given time. But here are my favorite:

Paint Tool Sai
Great for sketching and painting water-colory. 

Autodesk Sketchbook Pro
I love the marker tools in this!!!

Art Rage
Fantastic for crayons/chalk and oil paints

ALL of these are put through Adobe Photoshop CS6 before I post them, though, for cleanup, color tweaking, and (usually) texturizing. It turns this:

Into this:

Thanks for the question! I hope that helps. 😀