photoshop is good - if you can get an old copy or a pirate - because it has a wide range of tools, and its the industry standard.
clip studio paint is popular on here, but i have no experience with it.
paint tool sai is very easy to pirate, and is made for drawing so its good for comics. its vectors and lettering are.... nonexistent.. but if you can live with that its good. i use sai personally.
firealpaca is a free, apple-friendly program thats very similar to sai.