Well, I'll have to say Kazaa. I've used Morpheus and AudioGalaxy, but I find Kazaa is really the best for me. So far it has found almost everything I need (which is mostly music).

I know it's got craploads of spyware (damn thing opens up some online casino link every now and then), but Morpheus now has spyware as well (see http://www.vnunet.com/News/1130284). If you are looking for just music, AudioGalaxy is good because it has some less-popular music that you might not find on Kazaa.

Greg