Photoshop is the indusrty standard, and probably the best image editing software, that said it's not the easiest to learn. Lightroom is gaining ground, although a different sort of application, it's really more of a raw converter, but is quicker and easier for basic editing large numbers of image (raw or otherwise)
Paint shop pro, is similar in many ways to photoshop but a lot cheaper, although it doesn't seem to have the following photoshop has.
Gimp is also similar-ish to photoshop but a free application.
Personally I use photoshop for the tricky editing, and lightroom for the bulk of weddings etc.
I could live without lightroom, but I couldn't manage without photoshop. Wayne