Paint Shop Pro... hands down. It can do nearly 90% of what Adobe Photoshop can do. It's Adobe compatible which means it can use an filter that Photoshop can. It has a low learning curve, and is 10 times cheaper than Photoshop is.

"It is good to see ourselves as others see us. Try as we may, we are never
able to know ourselves fully as we are, especially the evil side of us.
This we can do only if we are not angry with our critics but will take in good
heart whatever they might have to say." - Ghandi