Photoshop is probably the most widely used, but I have had a hard time with it, it seems like a class would really be helpful.

I have been using Paintshop Pro at home it is way cheaper than Photoshop and I can do a lot of cool stuff with it. I was able to crop out a cousin one time from a picture of the kids we used on a Christmas card, and no one could tell (except the cousin). I use AutoCAD every now and then and understanding layers and layer control made it way easier for me to understand right off the bat.

Link to Trial Download