This is another retrospective post - I've done these when I was getting familiar with ArtRage.
ArtRage is a painting software, oriented towards replicating the traditional media. It mimics such paint qualities like wetness or volume, allowing for really realistic effects.
I use it most of the time, as it's also very straightforward to use. No need to spend time on set up - I can launch it and start drawing immediately - just like with pencil and paper.