You know that's a great question. I think the answer really depends on you. What type of art do you think you'll be using the editor for?
I'm partial to Painter 7, because I'm fasinated by the mimicing of traditional mediums.
Also ask yourself whether or not you're going to use things like scanned images or a wacom tablet etc... Each program has its strong points for handling these different types of media. i.e. photoshop is fantastic at photo manipulation.
Good luck.. And try to download some trial versions of these softwares. You'll be the best judge.
From,
Ben