>>6686350more than likely photoshop, though a lot of artists swear by Mangastudio (usually for less "painterly" things though it seems, stuff like uh well manga and also webcomics). Some people use artrage and that one that's on sale on steam (can't remember the name).
honestly it's just a matter of preference. Can it create a high-res image? do you like the brush options available? then use it. it's 2016 so all programs are free too
