ICQ was the best of all, but it died so many years ago.
SK's NateOn was good for transmitting large files which was virtually impossible with MSN Messenger, but more than half of Westerners seemed to use MSN Messenger and more than half of South Korean office workers seemed to use MSN Messenger only.
Skype was a little bit better for video communication, but the interface seemed ugly and fast text chatting seemed inconvenient. Since Microsoft's acquisition of Skype, I think it's being fast forgotten even by Microsoft itself.
Tencent's WeChat has 460 million. WhatsApp 300 million. NHN's Line 260 million. Kakaotalk 120 million. The world's largest non-mobile messenger is either Facebook, Yahoo Messenger, AIM, MSN/Skype, or Tencent's QQ. Tencent's market value is a little over US$100 billion. WhatsApp was started by two former Yahoo employees and is wanted by both Facebook and Google. Kakaotalk was started by some fomer employees of NHN of Naver which in turn was part of Samsung. Line is a product of NHN.
Numbers are important because they mean advertising income. The founders of WhatsApp will not sell Whatsapp at the price of Skype when it was sold to Microsoft. WeChat and Line were started by large IT companies in 2011. WhatsApp and Kakaotalk were started by former engineers of large IT companies in 2010 and 2009 respectively. At first, most Western users seemed to use WhatsApp. Most Chinese users chose WeChat. Most South Korean users chose Kakaotalk. Most Japanese users chose Line. That's no more true. There are more than 100 million non-Chinese WeChat users and more than 200 million non-Japanese Line users. Two thirds of Kakaotalk users live outside South Korea.
Most non-mobile messengers by now support iOS, Android, and Windows Phone. Likewise, most mobile messengers support Windows, Linux, and Mac OS.