Depends of what you want to do with it. GIMP is good as a general-purpose image editor, but is not really specialized into artworks creation. Krita, on the other hand, is.
MyPaint has some interesting qualities too (such as infinite canvas) didn't used it for a while though. It is for artists who like to do everything themselves, without relying too much on the software.
Inkscape may be for you if you're into vector / staying close to the show's style