aMSN, all my contacts use MSN. aMSN has a wonderful interface and features. it took me all of 3 seconds to convince a devout windows user to switch to aMSN from windows live messenger.
i also like Pidgin due to it's simple and universal nature, the spell checking is an interesting if some what redundant feature for an IM client.
in the past GAIM was a good friend to me and like Pidgin, with a plugin or two it can do just about anything. if i remember correctly Pidgin is a fork of GAIM?