It really depends on the artist and their personal tastes. If you are asking me to help you choose what to use, I suggest you just download trials of each one and find which suits you the most.

For me, my primary tool is Photoshop. I would love to use Corel Painter, but with past computers the program always crashed (I've yet to try it on the new laptop I got a couple days ago). Paint Shop Pro used to be my go-to software before I discovered Photoshop. Gimp's interface is too cluttered and I've never heard of Sketchbook Pro.