AIM was my first one too, now I use MSN and Yahoo the most. Trillian helps me use all three for support though so that's nice Keeps my customers happy too cause they can talk through any of them.
My first one was actually icq and after that followed by YM until today. Did tried before MSN in between but wasn't attracted to it.
I think I started on ICQ. Then I must have started using Yahoo or MSN or both, because eventually I got rid of all that and started using Trillian. Then I ended up with most of my contacts being on Yahoo, so I just started using that instead of Trillian.
My first messenger was yahoo. I am using only yahoo and gtalk. Actually rarely using them due to work load. DON.
My first encounter with meseengers is with Yahoo. I still prefer to use it over other messengers today.