Just out of interest really. Seems that a lot of people in the US use AIM, and a lot of people in the UK use MSN Messenger, from my experience. If you use more than one, put the network you use the most. Edit: Oh, and what client do you use, if it's a non standard one?
MSN only because it tells you on your desktop when you receive mail in your hotmail account and tells you who the sender is.