Which??

[in.essensce]

Which is the best software for digital painting??

[Seedling]

Try reading up on PhotoShop and Painter here http://conceptart.org/forums/forumdisplay.php?f=23 and here http://conceptart.org/forums/forumdisplay.php?f=78

If you’re on a budget, try the Gimp.

[Coen]

I think the most used apps, like Seedling said, are Photoshop and Painter, but it's really a personal thing. If you're on a budget, I heard Art Rage (http://www.ambientdesign.com/artrage.html) is pretty good too. I don't use it myself but I read good things about it.