GIMP is free, and I'm stuck using it right now because I messed up my copy of Paintshop Pro 8. (Will be reinstalling soon.)
In terms of "best", most people say Photoshop. I prefer Photoshop myself. The only thing that bothers me about it are some of the right-click and copy/paste options; this is probably because it works differently than in Paintshop Pro.
Paintshop Pro (I think it's on ver. 10 or 12 now) is much cheaper than Photoshop. It can do most of the things most people need a photoediting program to do. However, you do get what you pay for, and some of the things I'm learning in Graphic Arts take more effort to do in Paintshop Pro than in Photoshop.