uhh trillian has every messanger installed into it I believe, except maybe Xfire . When I used trillian a few months ago it had MSN, ICQ, AIM, and i think one or two others in it.
Xfire is pretty good/fun if you like playing games . but no one is going to have an account with it in real life that you meet probably, unless they play games.
I use MSN as well as almost everyone does.. doesnt have as good of smilies, but it has a lot more of them as well as custom display pictures and display names.
Overall i'd say AIM and MSN are most commonly used now-a-days so you might want to look into msn...