I don't have one, and the only 2 I have in memory right now, are ones I never much held to, and have since moved way beyond where they sort of struck me, back in the mid 90's.

I am interested in well done, at times monumental works, especially in history, so...

First one is by Edward Gibbon, in Book 3 of Volume I of The History of the Decline and Fall of the Roman Empire, again one I wasn't really attached to, especially since it is over 225 years old, and because it is man oriented, but, considering it was another of those tucked away monumental sayings in a monumental work, it struck for a time:

"If a man was called upon to name that period in the history of the world, where the human race was most prosperous and happy, he would without hesitation name that which elapsed from the death of Domitian to the accession of Commodus."

For a little more, perhaps more relative perspective, if you saw Gladiator, Commodus was the son and then emperor, who succeeded upon the death of Marcus Aurelius.

I have spent a lot of time studying roman, western, european, world history, so, all this is some of what the where I was coming from, was...

...and another was by Jacob Burckhardt, one a little more ironic, and yet one that I never attached to, and one I have never been able to relocate, but is in either The Civilization of the Renaissance in Italy, or in The Age of Constantine the Great:

"There is nothing more dangerous to the state than a young man with leisure."

and for that matter, just for kidding the logic further, a young woman, too...

More to come over time...