Painter has become a beast of a program. I still have the Paint Can it use to come with although I upgraded several time since (2015).
The particle brush is quite fun to use. I did a logo of a running horse lately and the blur effect was done with the particle brush.
Other than Photoshop, Painter has been my digital tool creation for years. ArtRage and Sketchbook pro are not bad but I find myself back to Photoshop/Painter for heavy duty work most of the time.
On the free side GIMP and Krita are also fun to use. inkscape, a vector painting program, is also a good open source program to try.
https://inkscape.org/en/