I use yahoo and aol IMs both ...but neither that much. I'm training my friends to email me rather than IM. I feel compelled to stop everything and answer an IM and when I'm working it's often a pain.
Considering the fact that here lately, I have been having alot of trouble with MSN I would gladly say aim. I think that after so long of being disconnected and bumped off you get to the point where you just want something new and I changed over to aim and have loved it ever since.
Hotwriter, I've been having trouble connecting to MSN messenger as well. I prefer MSN just because it gives you pop ups letting you know that you have mail. I think I have too many friends on my AIM that it makes signing on too much of a hassle, cuz everyone wants to talk all at once.
I use msn, aim, yahoo, icq pretty much everything for every different thing. msn i use for work online, aim is for personal friends, yahoo is for both, icq is for both. i also use skype too for business alittle bit.