I've tried them all, and there really is no winner. These protocols offer mostly the same features. I was not impressed with any of them, especially the ones with ads. Everyone has their own preference, so why stick to one? Makes no sense to have to run multiple IM clients...
I use a multi-protocol instant messenger (able to connect to AIM, MSN, ICQ, IRC, etc.) like Miranda-IM, Pidgin, or Kopete. Best IM programs Ive used.