You could take almost anything from this page. A few that ring especially true:
◾“Misers get up early in the morning; and burglars, I am informed, get up the night before.” – Tremendous Trifles
◾“The act of defending any of the cardinal virtues has today all the exhilaration of a vice.” – A Defense of Humilities, The Defendant, 1901
◾“A dead thing can go with the stream, but only a living thing can go against it.” – The Everlasting Man, 1925
◾“Fallacies do not cease to be fallacies because they become fashions.” – ILN, 4/19/30
◾“What embitters the world is not excess of criticism, but an absence of self-criticism.” – Sidelights on New London and Newer New York
◾“He is a [sane] man who can have tragedy in his heart and comedy in his head.” – Tremendous Trifles, 1909
◾“Moderate strength is shown in violence, supreme strength is shown in levity.” – The Man Who was Thursday, 1908
◾“The simplification of anything is always sensational.” – Varied Types
◾“Customs are generally unselfish. Habits are nearly always selfish.” – ILN 1-11-08
◾“The person who is really in revolt is the optimist, who generally lives and dies in a desperate and suicidal effort to persuade other people how good they are.” – Introduction to The Defendant
◾“To have a right to do a thing is not at all the same as to be right in doing it.” – A Short History of England, Ch.10
◾“All the exaggerations are right, if they exaggerate the right thing.” – “On Gargoyles,” Alarms and Discursions
◾“The comedy of man survives the tragedy of man.” – ILN, 2-10-06
◾“When learned men begin to use their reason, then I generally discover that they haven’t got any.” – ILN, 11-7-08
◾“The free man owns himself. He can damage himself with either eating or drinking; he can ruin himself with gambling. If he does he is certainly a damn fool, and he might possibly be a damned soul; but if he may not, he is not a free man any more than a dog.” – Broadcast talk 6-11-35
◾“The reformer is always right about what is wrong. He is generally wrong about what is right.” – ILN, 10-28-22
◾“Reason is always a kind of brute force; those who appeal to the head rather than the heart, however pallid and polite, are necessarily men of violence. We speak of ‘touching’ a man’s heart, but we can do nothing to his head but hit it.” – Charles II, Twelve Types
◾“Man is always something worse or something better than an animal; and a mere argument from animal perfection never touches him at all. Thus, in sex no animal is either chivalrous or obscene. And thus no animal invented anything so bad as drunkeness – or so good as drink.” – Wine When it is Red, All Things Considered
◾“When we step into the family, by the act of being born, we do step into a world which is incalculable, into a world which has its own strange laws, into a world which could do without us, into a world we have not made. In other words, when we step into the family we step into a fairy-tale.” – Heretics, CW, I, p.143
◾“A thing may be too sad to be believed or too wicked to be believed or too good to be believed; but it cannot be too absurd to be believed in this planet of frogs and elephants, of crocodiles and cuttle-fish.” – Maycock, The Man Who Was Orthodox
◾“Do not enjoy yourself. Enjoy dances and theaters and joy-rides and champagne and oysters; enjoy jazz and cocktails and night-clubs if you can enjoy nothing better; enjoy bigamy and burglary and any crime in the calendar, in preference to the other alternative; but never learn to enjoy yourself.” – The Common Man
◾“Do not look at the faces in the illustrated papers. Look at the faces in the street.” – ILN, 11/16/07
◾“The whole curse of the last century has been what is called the Swing of the Pendulum; that is, the idea that Man must go alternately from one extreme to the other. It is a shameful and even shocking fancy; it is the denial of the whole dignity of the mankind. When Man is alive he stands still. It is only when he is dead that he swings.” – The New House, Alarms and Discursions
◾“To hurry through one’s leisure is the most unbusiness-like of actions.” – “A Somewhat Improbable Story.” Tremendous Trifles
◾“There have been household gods and household saints and household fairies. I am not sure that there have yet been any factory gods or factory saints or factory fairies. I may be wrong, as I am no commericial expert, but I have not heard of them as yet.” – ILN, Dec 18, 1926
◾“Over-civilization and barbarism are within an inch of each other. And a mark of both is the power of medicine-men.” – ILN, 9-11-09
◾“By experts in poverty I do not mean sociologists, but poor men.” – ILN, 3/25/11
◾“Women have a thirst for order and beauty as for something physical; there is a strange female power of hating ugliness and waste as good men can only hate sin and bad men virtue.” – Chesterton on Dickens
◾“Marriage is a duel to the death which no man of honour should decline.” – Manalive
◾“Idolatry is committed, not merely by setting up false gods, but also by setting up false devils; by making men afraid of war or alcohol, or economic law, when they should be afraid of spiritual corruption and cowardice.” – ILN, 9/11/09
◾“I say that a man must be certain of his morality for the simple reason that he has to suffer for it.” – ILN 8/4/06
◾“To the humble man, and to the humble man alone, the sun is really a sun; to the humble man, and to the humble man alone, the sea is really a sea.” – Heretics, CW I, p128
◾“Great truths can only be forgotten and can never be falsified.” – ILN, 9-30-33
◾“The first two facts which a healthy boy or girl feels about sex are these: first that it is beautiful and then that it is dangerous.” – ILN, 1/9/09
◾“I have little doubt that when St. George had killed the dragon he was heartily afraid of the princess.” – The Victorian Age in Literature
◾“Art, like morality, consists of drawing the line somewhere.” – ILN, 5/5/28
If you are getting the impression that GKC is the most quotable writer since or including Shakespeare, you're right.