My favorite exchange is still:
LADY BRACKNELL: It really makes no matter, Algernon. I had some crumpets with Lady Harbury, who seems to me to be living entirely for pleasure now.
ALGERNON: I hear her hair has turned quite gold from grief.
And then second:
JACK: I have lost both my parents.
LADY BRACKNELL: To lose one parent, Mr. Worthing, may be regarded as a misfortune; to lose both looks like carelessness. And of course it's lines like these that makes the play hold up, no matter how many years pass.