I can't believe no one's said Yahoo yet. BY FAR THE VERY VERY BEST is Yahoo's instant messaging system. I've used AIM, ICQ and MSN and this is the best hands down. They have a lot better features (go to avatars.yahoo.com - they are sweet), are a lot more user-friendly and has a nice-to-look at and clean interface. AIM always has looked so prehistoric.
Whatever you do..PLEASE PLEASE PLEASE PLEASE PLEASE PLEASE PLEASE go with Yahoo. haha I am a huge Yahoo fangirl. And Yahoo has a lot of users on there too. Usually most folks have one for AIM and another for Yahoo.
I've never once had any problems with Yahoo! even though I have a painfully old and slow emachines POS.
Seriously, download it and give it a try. I PROMISE YOU you will never go back.
UPDATE: Yahoo is planning on allowing users to IM folks with MSN:http://messenger.yahoo.com/partners_...3B9W9uu6lwMMIF