Well there are many out there. There is free programs and the paid ones. Gimp and Paint.net is free but they are not as good as Photoshop or Coral draw (I think that is what it is called)
Gimp:
pros: Free and more tools than Paint.net
cons: very slow and you have to export to save
Paint.net
pros: Free and very easy to use
cons: Not many options and (for me) colour recognition is bad
Photoshop
pros: One of the best, Many tools and options to use
cons: Takes up a lot of space, Have to pay, Hard to work with
Coraldraw
Never actually used it.
Overall I think Photoshop is the best.