I've always been a fan of Paint Shop Pro myself, but that's really just product loyalty I think. I don't know enough about image editing to argue which one is the best.
SYG showed me a lot of things that Photoshop could do, and challenged me to do the same in PSP(not playstation portable SYG). Of course I couldn't. I wasn't sure if PSP could do those things or not, but he sure did make Photoshop look a lot better knowing how to use it and all