AIM. I'll use MSN, but I far prefer AIM over it. For one, my school's connection (the one I tend to be on most of the time now) gets a bit cranky at times; when it gets like that, AIM tends to stay connected for a bit longer than MSN will (and will actually reconnect; there are times when MSN absolutely refuses to connect for me, while AIM will). Two, I have more people I keep in touch with over AIM.
Plus, MSN laaaags this computer at times - it's rather annoying, really.