With GIMP you get great software considering that you don't have to pay a single penny for it, and it's a pretty decent graphics editor all together. Still, I'll have to go with Photoshop on this one. Photoshop is way to expensive, but then again it is a really powerful piece of software where you can do pretty much anything with a picture, and create pretty much anything out of nothing. The UI is also amazing. I've used Photoshop since 2008-09, and I probably will keep doing so for the next 10-20 years.
Even though GIMP is usually described as the free photoshop I have never used it. I use photoshop instead. If I had heard about GIMP before I used photoshop I would have definately tried this. GIMP might be good but with its floating screens, it looks silly.