My favorite program is Photoshop. I use Photoshop 7.0. It does take a while to learn and can be intimidating to some users. I know lots of people here use Photoshop Elements which does a lot of the same things as Photoshop, but is a bit easier to use.
One tip would be to download one of the free trials before you buy. Regular Photoshop can be pricey, whereas Elements is quite a bit cheaper.
If you have a MAC, I think Elements 5.0 works on that.