I have many favourites...mostly on ethics as I think about them. I love the one Robert has quoted.
Here's a few:
The shortest and surest way to live with honour in the world, is to be in reality what we would appear to be; and if we observe, we shall find, that all human virtues increase and strengthen themselves by the practice of them.
True wisdom comes to each of us when we realize how little we understand about life, ourselves, and the world around us.
Whom do I call educated? First, those who manage well the circumstances they encounter day by day. Next, those who are decent and honorable in their intercourse with all men, bearing easily and good naturedly what is offensive in others and being as agreeable and reasonable to their associates as is humanly possible to be... those who hold their pleasures always under control and are not ultimately overcome by their misfortunes... those who are not spoiled by their successes, who do not desert their true selves but hold their ground steadfastly as wise and sober -- minded men.
Do unto others as you would have them do unto you.
That version is attributed to Jesus...but it's there in most religions and lots of ethical systems. (Bearing in mind you sort of have to make sure they don't abhor what you would have them do unto you!)
Here's a few other versions:
"Ethic of Reciprocity" passages from various religions: Bahá'í Faith to Judaism:
"Ascribe not to any soul that which thou wouldst not have ascribed to thee, and say not that which thou doest not." "Blessed is he who preferreth his brother before himself." Baha'u'llah
"And if thine eyes be turned towards justice, choose thou for thy neighbour that which thou choosest for thyself." Epistle to the Son of the Wolf. 1
Brahmanism: "This is the sum of Dharma [duty]: Do naught unto others which would cause you pain if done to you". Mahabharata, 5:1517 "
"...a state that is not pleasing or delightful to me, how could I inflict that upon another?" Samyutta NIkaya v. 353
Hurt not others in ways that you yourself would find hurtful." Udana-Varga 5:18
"Therefore all things whatsoever ye would that men should do to you, do ye even so to them: for this is the law and the prophets." Matthew 7:12, King James Version.
"And as ye would that men should do to you, do ye also to them likewise." Luke 6:31, King James Version.
"...and don't do what you hate...", Gospel of Thomas 6. The Gospel of Thomas is one of about 40 gospels that circulated among the early Christian movement, but which never made it into the Christian Scriptures (New Testament).
"Do not do to others what you do not want them to do to you" Analects 15:23
"Tse-kung asked, 'Is there one word that can serve as a principle of conduct for life?' Confucius replied, 'It is the word 'shu' -- reciprocity. Do not impose on others what you yourself do not desire.'" Doctrine of the Mean 13.3
"Try your best to treat others as you would wish to be treated yourself, and you will find that this is the shortest way to benevolence." Mencius VII.A.4
"Do for one who may do for you, that you may cause him thus to do." The Tale of the Eloquent Peasant, 109 - 110 Translated by R.B. Parkinson. The original dates to circa 1800 BCE and may be the earliest version of the Epic of Reciprocity ever written. 2
This is the sum of duty: do not do to others what would cause pain if done to you. Mahabharata 5:1517
Islam: "None of you [truly] believes until he wishes for his brother what he wishes for himself." Number 13 of Imam "Al-Nawawi's Forty Hadiths." 3
"Therefore, neither does he [a sage] cause violence to others nor does he make others do so." Acarangasutra 5.101-2.
"In happiness and suffering, in joy and grief, we should regard all creatures as we regard our own self." Lord Mahavira, 24th Tirthankara
"A man should wander about treating all creatures as he himself would be treated. "Sutrakritanga 1.11.33
"...thou shalt love thy neighbor as thyself.", Leviticus 19:18
"What is hateful to you, do not to your fellow man. This is the law: all the rest is commentary." Talmud, Shabbat 31a.
"And what you hate, do not do to any one." Tobit 4:15 4