I think the MyPaint brush set might be my favorite brush set. It has just about everything you need for digital painting. No need to hunt down 10 different brush packs to get what you want when MyPaint does it all. Also being a library as well, it's integrated into GIMP when installed and is available for other programs like krita or OpenToonz/Tahoma2D. Speaking of OT or T2D, I find it to be a great program packed with features. And it uses primarily the MyPaint brushes, which is a plus in my opnion.