I use Paint Shop Pro, myself, because I have better things to do with my money than send it all to Adobe.

The function I could not live without? I doubt I could narrow it down to just one. The clone and smudge tools, blur commands, and various plugin filters (both freeware and commercial) are all essential to my work.

I highly recommend Alien Skin's Eye Candy plugin, which includes a number of very useful filters (HSB noise, fire, smoke, fur, jiggle, gradient glow, etc.). Their Xenofex plugin looks quite helpful, also, although I haven't bought it yet.

And I use the free MuRa filters extensively.