ICQ

Pros: It's been the best client for years, attracting the most people.
Cons: They recently sold out, and didn't look carefuly at what users wanted.

Recently ICQ removed the possibility of posting Chat Requests (your name and a few lines about yourself or what you wanted to talk about). This was easily the best feature ICQ had over the competition. If after an outpouring of frustration from dedicated users, they refuse to reinstate this option I think ICQ will quickly fall from it's former glory.

If you are an ICQ user who has used the Chat Request feature in the past, please see our poll and add your support.
http://www.icq.com/boards/view_messages.php?tid=4824&topic_id=621043

If you're a new user looking for the best instant messaging client to meet new people, I never though I'd say this but ICQ is no longer the one.