i always used IRCle back in the day, but it's a pay program. Very well-featured tho, i used it because of quick access to IRCop and moderator controls.
I use colloquy 2.0 beta. I like it a lot but I get a beachball when I go to very populated channels. (1000+ people on with many posts per second usually on file sharing ones) I think I might try ircle now.