I'm quoting these from memory, after having been involved in two productions of the show (one for an entire summer), so they may not be word for word. However, you can point to any of Algernon's lines as a funny quote or conventional wisdom turned upside down.
Jack: I have come to town expressly to propose to [Gwendolen].
Algernon: You said you were in town for pleasure! I call that business.
Lady Bracknell: I have always been of opinion that a man who desires to get married should know either everything or nothing. Which do you know?
Jack: I...know...nothing, Lady Bracknell.
Lady Bracknell: Good. I'm glad to hear it. I do not approve of anything that tampers with natural ignorance. Ignorance is like a delicate exotic fruit; touch it and the bloom is gone.
Algernon: All women become like their mothers. That is their tragedy. No man does. That's his.
Gwendolen: I never travel without my diary. One should always have something sensational to read in the train.
Cecily: Oh, I don't think I would care to catch a sensible man. I shouldn't know what to talk to him about.
Jack: Gwendolen, it is a terrible thing for a man to find out suddenly that all his life he has been speaking nothing but the truth. Can you forgive me?
Gwendolen: I can for I feel you are sure to change.