Definately MySapce.
I couldn't vote for Vista or ME because although they look crap, I have never used them.
I was also tempted to vote for MSN/AIM but those arn't actually that annoying when you use a decent client (i.e. BitlBee). If you use the official client, then that is bad, e.g. MSN Messenger 7 is a horrible piece of software.
MySpace is definately very annoying. It can be fine when used appropriately, but not with all the horrible themes that some people use.
