Depends on what you do & the size of your wallet. I use Photoshop exclusively, but then I've never had to pay for it.
Corel Painter is a great option which is far more affordable and very high in standard as well. Keep in mind though that Painter is at its core a paint-application. (Of course you can still use it as photo-editing software, just like Photoshop, though.) It all depends on what you are working with. If you are heavily into photo editing the best choice is Photoshop. If you are into artistic creative work, Painter is superior. Naturally, some people like to have both on hand for different purposes.
Jasc
Paint Shop Pro is a good option for photo editing and such and is nearly 1/9th the price of Photoshop.
Then there's
the Gimp, which is a free, open-source option.