I made my millions using pro tools and autotune-oh, wait, we're talking about image software!
I lol'd!
i use photoshop, its simply the best, but as many have said, its expensive if you buy it
the truth is though, any program you use has a learning curve, once you figure it out you'll be smooth sailing, so you cant go wrong with all the free suggestions people have made like GIMP! if you dont have to spend money, why would you? check out the free stuff and if you feel you need "more" then you have those other options
for character battle, dialogue and reaction faces, i recommend using a vector based program like adobe illustrator, or maybe manga studio and anime studio- so you can color them easily and rescale them without loss of quality