Irssi is my favourite for linux and Windows.
It is so easy to use, and still advanced
It has many features, and it's never disturbing you while you do something else.
Scripting is also nice, but I don't use scripts so much that I could comment anything on Irssi's scripting-features.
The commandline/text-only interface is a pure ++ for me, becouse the client is light, and can be run in a text-terminal while you play games or watch movies.
Yet another good side with text-interface is that you do not need any special tools to use it remotely. It is just such a painful time to try to get a graphical remote-use interface working...