MSN hands down, AIM is total crap...yahoo is like MSN but not as good....google talk is awful I like MSN for it's interface, features and the fact most of my friends have it. Stability wise, it's not the best but you can't have everything!
I like MSN the best, only because it has a lot of patches / mods out for it and because thats what my friends use. There is no use chaining to another IM if I cant keep my contact list.
Skype almost exclusively.
Sometimes MSN when the person I want to talk to has no Skype.
Plus sometimes Miranda for a bunch of less popular IMs (plugins).