you can try all of them,then choose the one best for u!

I prefer MSN for study, for precisely the reason that Harregarre mentions, also, on MSN you can activate your microphone without requiring a two way conversation to start. Skype requires that both parties speak. Sometimes it is good to practice by reading text aloud as another person types it. Though you can do this with Skype, language partners that don't have a microphone or cannot use one at the time would still be able to hear you on MSN easily, with skype it is a little harder. I can't say anything for Yahoo.

It depends on your microphone but I think Skype does a good job anyway. MSN has one advantage though, it has a short voice message function, so you can record your message and the other person can play it as many times as he/she likes. Which is quite handy if you're learning a new language because you might have trouble understanding the sentence if you hear it just once. I can't say anything about Yahoo Messenger though, because I don't use it.