Question: Given this reference text, what was John Dalberg-Acton's most famous pronouncement, and to whom did he make it?
Evidence: John Emerich Edward Dalberg-Acton, 1st Baron Acton, 13th Marquess of Groppoli, KCVO, DL (10 January 1834 – 19 June 1902), better known as Lord Acton, was an English Catholic historian, politician, and writer. He is best remembered for the remark he wrote in a letter to an Anglican bishop in 1887: 

"Power tends to corrupt, and absolute power corrupts absolutely. Great men are almost always bad men…"

In 1870, along with his mentor Döllinger, Acton opposed the moves to promulgate the doctrine of papal infallibility in the First Vatican Council, travelling to Rome to lobby against it, ultimately unsuccessfully. Unlike Döllinger, Acton did not become an Old Catholic, and continued attending Mass regularly; he received the last rites on his deathbed. The Catholic Church did not try to force his hand. It was in this context that, in a letter he wrote to scholar and ecclesiastic Mandell Creighton, dated April 1887, Acton made his most famous pronouncement:

But if we might discuss this point until we found that we nearly agreed, and if we do agree thoroughly about the impropriety of Carlylese denunciations and Pharisaism in history, I cannot accept your canon that we are to judge Pope and King unlike other men, with a favourable presumption that they did no wrong. If there is any presumption it is the other way, against the holders of power, increasing as the power increases. Historic responsibility has to make up for the want of legal responsibility. Power tends to corrupt, and absolute power corrupts absolutely. Great men are almost always bad men, even when they exercise influence and not authority, still more when you superadd the tendency or the certainty of corruption by authority. There is no worse heresy than that the office sanctifies the holder of it. That is the point at which the negation of Catholicism and the negation of Liberalism meet and keep high festival, and the end learns to justify the means. You would hang a man of no position like Ravaillac; but if what one hears is true, then Elizabeth asked the gaoler to murder Mary, and William III of England ordered his Scots minister to extirpate a clan. Here are the greatest names coupled with the greatest crimes; you would spare those criminals, for some mysterious reason. I would hang them higher than Haman, for reasons of quite obvious justice, still more, still higher for the sake of historical science.

Thenceforth he steered clear of theological polemics. He devoted himself to reading, study and congenial society. With all his capacity for study, he was a man of the world and a man of affairs, not a bookworm. His only notable publications were a masterly essay in the Quarterly Review of January 1878 on "Democracy in Europe;" two lectures delivered at Bridgnorth in 1877 on "The History of Freedom in Antiquity" and "The History of Freedom in Christianity"—these last the only tangible portions put together by him of his long-projected "History of Liberty;" and an essay on modern German historians in the first number of the English Historical Review, which he helped to found (1886). After 1879 he divided his time between London, Cannes, and Tegernsee in Bavaria, enjoying and reciprocating the society of his friends. In 1872 he had been given the honorary degree of Doctor of Philosophy by the University of Munich; in 1888 Cambridge gave him the honorary degree of Doctor of Laws, and in 1889 Oxford the Doctor of Civil Law; and in 1890 he received the high academic accolade of being made a fellow of All Souls College, Oxford.
Answer: In a letter to Mandell Creighton, John Dalberg-Acton made his most famous pronouncement: "Power tends to corrupt, and absolute power corrupts absolutely. Great men are almost always bad men, even when they exercise influence and not authority, still more when you superadd the tendency or the certainty of corruption by authority."