All the kids here will say photoshop and I agree with them, but I started out with gimp and learnt mostly through that...
but if you want power and satisfication aswell as resources... its photoshop cs 1/2 or 3...
I use cs3... then there's paint.net, that has one of the best coded colour definining, infact, just get gimp2.2, paint.net, and photoshop, gimp2 and paint.net are free, photoshop is a couple hundred depending on what version you get.
they all have somthing diffrent from one another so like I said, get all of them, besides paint shop pro, hate the thing :S
No doubt'fully the simon cowell of the fpsc section :S