Marcus Tullius Cicero

Marcus Tullius Cicero was one of the greatest leaders of the Roman Republic. Over the course of his career, he became, among other things, an orator, lawyer, and politician. Also, for centuries, he was considered one of the greatest philosophers. One of his best-known works is On Duties. 

According to the Internet Encyclopedia of Philosophy,

Written in the form of a letter to his son Marcus, then in his late teens and studying philosophy in Athens (though, we can gather from the letters, not studying it all that seriously), but intended from the start to reach a wider audience. Cicero addresses the topic of duty (including both the final purpose of life, which defines our duties, and the way in which duties should be performed), and says that he will follow the Stoics in this area, but only as his judgment requires. More explicitly, the letter discusses how to determine what is honorable, and which of two honorable things is more honorable; how to determine what is expedient and how to judge between two expedient things; and what to do when the honorable and the expedient seem to conflict.

The book is as wonderful as it is dense and aphoristic. But my favorite and most personally influential line in the work describes his conception of the greatest thing to be praised.

He writes in Book One,  Section 88 that:

Furthermore, we should not listen to those who think we should be deeply angry with our opponents, and consider that that is what a great-spirited and courageous man does. For there is nothing more to be praised, nothing more worthy of a great and splendid man than to be easily appeased and forgiving. Among free peoples who possess equality before the law we must cultivate an affable temper and what is called loftiness of spirit.

Notably, this has been translated differently as followed:

Neither must we listen to those who think that one should indulge in violent anger against one’s political enemies and imagine that such is the attitude of a great-spirited, brave man. For nothing is more commendable, nothing more becoming in a pre-eminently great man than courtesy and forbearance. Indeed, in a free people, where all enjoy equal rights before the law, we must school ourselves to affability and what is called “mental poise”;/a for if we are irritated when people intrude upon us at unseasonable hours or make unreasonable requests, we shall develop a sour, churlish temper, prejudicial to ourselves and offensive to others. And yet gentleness of spirit and forbearance are to be commended only with the understanding that strictness may be exercised for the good of the state; for without that, the government cannot be well administered.

His remarks may apply to your personal life and may be summed in the following manner:

A great person is kind, gentle, easily forgiving, affable, and calm.