It depends. o_O; When talking to people I know IRL or else really good online friends, I prefer MSN. But since I know so many more people online than offline, I prefer to use AIM because it just seems... I dunno. You can't change your display name and its just easier to tell who's who at a quick glance. Plus, it has the custom away status.
I've sorta started using Trillian now, though. So I'm online on all of those. XP