Hmm, where to begin....

I use Photoshop. As jrw pointed out, it's powerful to do things like layers, layer masks, etc. Adobe Camera Raw can do many of the edits before you open the file into Photoshop. Adjusting a photo that is too bright, or too dark, can be accomplished with the graduated filter tool, for example.

I do a lot of landscape shoots, so these lend themselves to adjustments that layers, masks, "curves" and filters contained in Photoshop. Plus, I use several plug-ins, to stretch the capabilities of PS.

I do take a lot of portraiture, too, and PS does a great job on these. Lightroom does a great job for portraiture, where lighting conditions are similar. Many photos can be adjusted all a once. It works well for weddings, etc. A lot of these multiple adjustments can also be done in Bridge / Photoshop, by setting up "actions."

I try to stick to PS, because learning post processing is time consuming. I want to become proficient at it. I picked it because of that.

Other programs (Photomatix, PTGui, etc.) complement it nicely, too.