I go with AIM. I'll list my "Goods" about each though.
AIM ~ Chat rooms, file sending, DeadAIM (ph34r it!), away messages, the profiles, linking (if you like that stuff), easy to sign up
MSN ~ Uh, name changing. And that's about it for MSN... >>
YIM ~ I like in the new one that you can hide from certain people. And the voice chat system works GREAT.
ICQ ~ Don't even ask. Because there is none.
Now for the "bads"
AIM ~ It has a rather ugly look, but it is pretty easier skinned. Also non-hacked AIM has the ugly ads.
MSN ~ Convos like to hurt my computer... T_T, Windows Messanger sucks, File sending sucks, and it takes forever to sign up for. Plus you need the e-mail address for when you sign-up.
YIM ~ It's ugly, and no one uses it. >>;;
ICQ ~ SPAM! >.<;;;;;;;
Trillian is okay, but I don't like how it works. I'd rather have the Buddy Lists seperated, like how AIM does the linking. That's nice. :)
But if you ever want a real chat, go with IRC.