I've never used YIM mainly because not many people use it and I hate the application. When I was younger I used AIM most of the time but have since then grown and gotten stuck with Windows Live Messenger (MSN). I've used Trillian before, but I dislike the UI of it and ever since Google Voice came out, I haven't had any reason to use Skype.

So for me, it's always WLM and GTalk.