There's so many, but mostly I remember all the dialoges. E.g. the "I will marry you"-dialog,the "When my boy walks down the street"-dialog, the 308 "never to play music in my prescence again"-dialog and so on...
But one one-liner was the answer Brian gave Ted in season 5, when Ted beggen him to talk about Justin leaving and loosing Michael as a friend :
Ted: You can't fool me. You gotta free yourself of this burden. Release it. Let it all hang out.
Brian: My mother was a frigid **** My father was an abusive drunk. They had a hateful marriage, which is probably why I am unwilling or unable to form a committed long-term relationship of my own. The fact that I drink like a fish, abuse drugs, and have more or less redefined promiscuity doesn't help, much. As a result, I've lost the two people in my life that mean most to me.
And maaany many more