A few years ago, I wanted to learn about photo editing, so I took a several-week night-school course in Photoshop at a local high school. We all downloaded the trial version of Photoshop CS 5 for the purposes of the class. I liked it a lot, but my trial expired and I was really reluctant to buy a full version because of the cost. When it became available, I got the 'free' version of Photoshop CS2. I really, really like it. Like it's already been said: Photoshop is the standard by which everything else is judged. To be honest, though, I don't use it often enough to remember how it works. Lots of things to remember when it comes to masking off areas, adjusting levels, etc. Every time I crank it up, I have to Google up some brief tutorials to refresh my memory on how to do stuff.
Of course, I have a crappy memory, too.
If you're going to use it often enough, and if you have a decent memory for processes, seek out a class so you can learn the basics on how it works. Then make an effort to use it often enough that you stay fresh on things. Otherwise, go with something more user-friendly. I don't know what that "something" is, I'll have to rely on those more experienced than myself to offer suggestions.