Photoshop is sort of the standard, and good, but it's also quite expensive.
You may want to try The Gimp (which is free). It's very capable, but I personally dislike its interface (it may have improved lately, though - I haven't looked at it in a few years).
Paint Shop Pro is a good budget alternative. (Again, I haven't tried the latest version, but I used it for years before I got PS.)