X-Chat Aqua
After a difficult search X-Chat Aqua has won the coveted (in my mind, anyway) default IRC client spot on my MacBook.
Irssi is still my tool of choice but sometimes a GUI-based IRC client is helpful.
In the end Growl support and a complete set of independent network connection preferences (yay SOCKS proxies!) won me over.
Give Colloquy a try. I think that I like that better than x-chat.
I tried Colloquy out and it’s not a bad IRC app, it just didn’t have exactly what I was looking for. Mainly support for SOCKS proxies and full growl support (I could only get colloquy to send growl notifications for “activity in channel” whereas Xchat Aqua’s growl notifications actually show you the new messages).