There's just so many to choose from! But right this moment, I'm going to choose the signing of the Magna Carta in 1215. Cited by most authorities to be one of the earliest English (that is, of the English people, not the language, as it was written in Latin) documents which lay the foundation for constitutional law, the Magna Carta was the first document forced onto an English King by his subjects in an attempt to limit his powers by law. Many Americans, if they even know the Magna Carta exists, believe it to be something akin to our Constitution, but that isn't the case at all. In fact, England doesn't have a single Constitution. Rather, it has a collection of various legal documents which, together, form the basis of their government.
But anyway, it was revolutionary. A large part of Magna Carta was copied, nearly verbatim, from the Charter of Liberties of Henry I, issued when Henry I ascended to the throne in 1100, which bound the king to certain laws regarding the treatment of church officials and nobles, effectively granting certain civil liberties to the church and the English nobility. But this was voluntary on the part of Henry. In 1215, the Great Barons descended upon London and forced King John (younger brother of the late Richard Coeur de Lion (the Lionhearted)) to sign the expanded Magna Carta, which severely limited the King's power and authority. It was unheard of, and though the people were generally accepting, powers abroad, including the Pope, condemned the act. The Pope even released John from his oath to obey it (and in those days, if the Pope said something was so, it was the same as if God Himself said it).
As I said, it was revolutionary, and it changed the face of English history, and by extension, all of Western history. Our very own democratic republic is built upon many of the principles of the Magna Carta. Seeing this document signed in the meadow at Runnymede on June 15, 1215, would be an experience I would absolutely cherish.