Krita is my main and most used tool
“At Autodesk, we believe creativity starts with an idea. From quick conceptual sketches to fully finished artwork, sketching is at the heart of the creative process. You never know when a great idea will strike, so access to fast and powerful creative sketching tools is an invaluable part of any creative process. For this reason, we are excited to announce that the fully featured version of SketchBook is now FREE for everyone!”
I have ArtRage 5, upgraded from the Lite version that came with my Wacom Intuos Draw. I prefer Krita over ArtRage for sketching/drawing and digital painting in general. However, for imitating the look of real paints and color there’s ArtRage. ArtRage is the most affordable when compared to Corel Painter.