AIM with DeadAIM for most correspondence.
i use gaim for ICQ and one AIM account that i don't want to check often (but am consistently signed onto. usually, i'm idle for 250+ hours).
i use MSN for the few people abroad who don't use ICQ (and obviously are not going to be caught dead using AIM since that's really mostly used in america).
even if you want recommendations for a new instant messaging client (e.g. like you said by mozilla), you need to get users to subscribe to the service anyhow, and nobody really likes change unless forced upon them.