Besides plain old paper and pencil, my weapon of choice for art is Krita. It's got all the functionality I need and the price is perfect: FREE! I sometimes use ClipStudio Paint—the brushes are amazing—but I'm so accustomed to Krita's workflow, that I end up wandering back to Krita.
I'll definitely be expanding on this post more in days to come. Talking about art is fun. This is a placeholder while I work out how to implement a simple blog on this site.