Well, that is an impossible question to answer if I ever saw one!
There is no "best" photo editing program. There are many great ones though. The tough part is finding one that is right for you. Things to consider are price, features, organization, etc.
I can tell you there are a few popular ones including Adobe Light Room, Photoshop, Photoshop Elements, programs that come with cameras, Picasa, and GIMP.
For Macs you have Light Room, Photoshop, iPhoto, and Aperture.
You really need to figure out what you need in the editor before choosing the best one for you though. Do you want to selective color? Strong RAW image processing? Just increase/decrease saturation and make black and white photos? Do you want your editor to also organize your photos and add keywords? Do you want to invest the time necessary to learn how to get the full functionality out of the program or learn the basics you need?
Once you answer some of those questions we can hopefully point in the right direction for a program that will work for you.