MSN ftw, and I use IRC but only to find me some CS tournaments 
Im addicted to AIM. Everyone I know uses it, well…mostly everyone I know. =P If you want to add me; im: Kurock92
MSN. I ignore the fact that 85% of the visible area is taken up by ads.
In Ubuntu, Kopete. Because Gaim goesn’t work and besides just looks boring.
I did use Miranda for a bit. S’ok.
I’m hoping Opera has built in IM one day.
I started with IRC (no kidding), and then got dragged onto MSN by my friends, I think it was on version 3 at that time. Then because of other friends (mostly online), I got to try most of the other ones.
MSN is good, although a little bit bloated, and damn proprietary (actually, using Pidgin, Miranda, etc. is a violation of the terms of use).
Skype, I’d say, is excellent, but I wish they did more work on the IM part.
Yahoo is very good too, probably one of my faves.
Google Talk is the less bloated one but still needs moar features.
AIM is a hardcore CPU hog, needs refining.
IRC is getting old, but still the easiest to implement (and I’m a programmer,so… :-p ). It kinda needs a refresh, but I heard rumors on people attempting to enhance it, I hope I’ll see results.
Oh, well, I have no faves.
I decided to remove MSN and Gabber (a XMPP client, I used it for GoogleTalk and Live Journal talk) from my computer. So now I’m down to IRC and Skype, and I don’t even turn Skype on unless someone asks me to.
From where I stand, instant messengers are just a big waste of time. If I need to talk to someone, I prefer to do it either through actually talking (on the phone, in real life, on VoIP) or by mail I’m a big fan of old school mail systems, telegrams, and of course e-mail & pms. As for talking for the sake of talking, small talk, or when I need specific help, IRC is the best. Like Dok, I’m hoping for a new protocol to be developed with the same functionality of IRC, but for now, it’s been by far the greatest way of chatting and meeting people on the internet.
I use MSN. The first thing I ever used, I think, is Yahoo Messenger. Anyway, I prefer MSN. I never had problems with it and I never really have complaints against it. It’s never really disappointed me and I don’t know what more I could want from it. So… I choose MSN any day. Most of the people I know use YM though… and that’s a good thing… because then I can use YM for school stuff and things like that, while my MSN will still be reserved for ‘special’ people.