I've used Photoshop since the 1st version (when it didn't have a version number!), and I've also used a host of other programs on PC, Mac, and SGI. Although you'll run into a lot of used-this-first-like-it-best recommendations, there's really no comparison to Photoshop's HUGE feature set. It is a professional package however; if all you're after is niche editing/painting, you should look around carefully at smaller, more specialized software (like Jasc Paintshop Pro or Serif's Photo Paint).

Adobe Photoshop 7 is a bit expensive but that's what you pay for an industry standard.