Well, now adays you can use clients like Pidgin which allow you to use many IM services (ICQ, AIM, Jabber/GoogleTalk/FaceBook Messenger, Yahoo, Gadu Gadu, QQ, IRC, etc) within just one client and allows you to log conversations.

Personally, if I had to choose "the best" I'd lean towards Jabber, tons of decentralized servers, allows you to use your own email address if your server allows it, etc. My second favorite would be AIM though, it's secure, you don't get spammed to high hell like you would ICQ, but that's about it.
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