I myself was quite impressed with Krita (the new 3.0 release). Very impressive brush settings, and my favorute so far: the drawing assistants. The more intriguing features I mentioned in a short article with videos (if it could be of any help):
polydraw.com/blog/krita-highlights
Frankly I did not expect to find something so nicely developed from a free, open-source app. Thumbs up for their good work.