MSN. I've never really gotten into AIM (nor have any of my friends) -- from what I've seen it doesn't really offer any flexibility or customization that MSN does.
I just use yahoo too... Yahoo works for both yahoo and msn contacts, so there is no need for both. And the new Y!9 has some great new features...
My friends use just about everything, yahoo, myspace IM, MSN, AIM, so I use Trillian to message them all w/out having to download each messenger.
I'd say neither MS or AOL. Skype is the one I like. I won't work with an offshore developer unless they install it. Good IM, and good calls too.
AIM is my favorite instant messaging program. I use Skype for telephony apps, because AIM's voice support is crap.
Its a long time since I used either one of them. Currently my favorite is gtalk. Before it was yahoo and skype. Now ever since I founf Gtalk, thats it for me.