I think yahoo messenger in Russia - mail.ru agent in China - QQ in Sri Lanka - yahoo messenger in Vietnam - yahoo messenger
If you are talking about the whole world, then I think yahoo and msn are on the top, since many years. However skype is also speeding up as a good and reliable chatting messenger But if you ask me in my country, then its a local china messenger called QQ and msn are at their highest
Here's the list: Skype Live Messenger Gtalk Yahoo Messenger Xfire (For Gamers) They are all very much popular these days. Make sure that they are not arranged according to their popularity.
I was using Yahoo messenger for last ten years and I almost enjoyed all the features that was incorporated in it. But recently I have started using skype for online audio chat. It works great and better than ISD calls with telephony.
MSN has been the most popular for ages, but it could be that facebook has overtaken messenger in its popularity. Tho I still prefer messenger (As long as you don't use the new msn)