I use X-Chat Aqua, which gets the job done pretty well. Other choices include Colloquy, ircle, and Snak, and of course irssi. There's always the integrated clients in multi-protocol chat clients such as Fire and Adium X, too. Just for the record, http://www.google.com/search?q=IRC+client+for+mac
Just for the record my ass: I know I can google. I want to find out what the best if from someone here. I can google and I wouldn't be able to find the best user tested one.