I prefer using AOL Instant Messenger. Back in the day, I liked to use Windows Live Messenger with their official client, but I feel that they really dropped the bomb in their last major update. That, added to the fact that they don't support the service anymore (RIP. MSN Messenger), made me really switch to AIM using Pidgin.
Besides, over Instant Messenger, I contact maybe one or two people anyways. For anything else, I use Skype... But my favorite is historically MSN/AIM.