I'm having a hard time deciding. The latest Bearshare, Gnucleus, and Shareaza all work very well for me. Recently, I'm searching, downloading, and uploading to a good variety of other clients on all of them. except for Morpheus - which I have yet to download anything from no matter how hard I try. A friend of mine seems to have no trouble with that, so maybe it's just tough luck on my part?
I honestly have not tried Phex, though...although I will now feel compelled to