Photoshop
In my daily life as a digital photo artist I would never ever want to do without Photoshop again. For my pictures, I definitely place the greatest emphasis on post-editing with Photoshop.
I do, however, try to get most of the work done photographically first. Photoshop only enters into the equation at the stage where I cannot get any further with pure photography alone.
Because I know Photoshop well and am familiar with how it works and how to use it effectively, Photoshop always opens many more doors for my work than I will probably ever be able to enter.