When i was in high school, my favorite application back then was mIRC. It is an IRC client on Windows and i believe it's the best. Thanks to IRC, i finally met my ex girlfriend (now my fiancee) who also joined IRC at that moment. We had a talk in IRC and then decided to meet up and you know the rest
OK, back to mIRC. So in 2008, the development of mIRC has started to work on unicode support and finally two years later (which is this year), they have finally fulfilled their dreams of having unicode-supported application which should run better in Windows platform, including Windows 7. It also supports for speech, IPv6, UPnP, and many other features described in their Changes page.
Even though i didn't use mIRC again at the moment (as they don't have any Linux version of it), i still think mIRC is a great application and keep up the good work of creating the IRC client on Windows