mIRC is the best for chatrooms.
AIM is the best for filesharing conversations and for chatting with people who are using mobile phones. It was also the best for smilies, but the new ones are hideous and the option for small smilies has been removed (or I just can't find it ). Probably the best for user-friendliness, too.
Y!M is the best for one-on-one chats and voice conferences and, following the above bit, smilies.
Skype is the best (so I'm told) for webcams and one-on-one voice chats.
MSN is not the best for chatrooms, filesharing conversations, chatting with people using mobile phones, one-on-one chats, voice conferences, webcams or one-on-one voice chats. Or smilies. Or style. But it is the best for obnoxious usernames.

Not sure which of the top four come up trumps for style, now that I think about it.