Google Talk. On Gmail. With video. I hate MSN. Yahoo is decent but no compatibility between PC and Macs when it comes to video...yet, hopefully. AIM has been thwarted with ads for too long now. Skype is just...slow, stupid, seriously bad. And all the other ones are just compilations - that's original [sarcasm].
msn, because it's the only one people I know use. I don't know how people could try out different IM programs without having friends that use every IM program. I don't really see picking an IM program as a choice either.
Miranda IM & Pidgin. Light weight, I don't need video or Audio chat really, plus they are multi Instant Messenger Clients so I can chat on just about any network such as MSN,AIM,Yahoo,Jaboo,ICQ, IRC, etc. It also elimiates any reason to pick any one network over the other according to what my friends use. A good question is also, which network do you use. As some programs allow for multi logins and networks.