Winston Churchill (1874-1965) said before the House of Commons on November 11, 1947:
 
“Many forms of Gov­ern­ment have been tried, and will be tried in this world of sin and woe. No one pre­tends that democ­racy is per­fect or all-wise. Indeed it has been said that democ­racy is the worst form of Gov­ern­ment except for all those other forms that have been tried from time to time.”
 
“Winston Churchill’s paradox—that democracy is the worst form of government, except for all the others” was cited in print in 1951. Churchill prefaced his remark with “it has been said,” but it’s not known who said the popular saying about democracy before 1947.
 
   
Wikipedia: Winston Churchill
Sir Winston Leonard Spencer-Churchill, KG, OM, CH, TD, PC, DL, FRS, Hon. RA (30 November 1874 – 24 January 1965) was a British politician, best known for his leadership of the United Kingdom during the Second World War. Widely regarded as one of the greatest wartime leaders of the 20th century, he served as Prime Minister twice (1940–45 and 1951–55). A noted statesman and orator, Churchill was also an officer in the British Army, a historian, a writer, and an artist. He is the only British prime minister to have received the Nobel Prize in Literature and was the first person to be made an Honorary Citizen of the United States.
 
18 September 1951, Canton (OH)

, “Democracy the Sum of Its Part” (editorial), pg. 6, col. 1:
WILLIAM Henry Chamberlain has undertaken for The Wall Street Journal a discussion of the weaknesses of democracy. His enterprise is tackled in the spirit of Winston Churchill’s paradox—that democracy is the worst form of government, except for all the others.
 
Google Books
Shame & Glory of the Intellectuals
By Peter Viereck
Boston, MA: Beacon Press
1953
Pg. 245:
“Democracy is the worst form of government, except all those other forms that have been tried from time to time.” — WINSTON CHURCHILL
 
14 April 1954, Trenton (NJ) Evening Times, “Quotable Quotes,” pg. 22, col. 4:
“It has been said that democracy is the worst form of government except all those other forms that have been tried from time to time.”—Sir Winston Churchill.

Richard M. Langworth (Churchill historian, automotive and travel writer)
“Democracy is the worst form of Government…”
by RICHARD M. LANGWORTH on 26 JUNE 2009
(...)
Churchill said it (House of Com­mons, 11 Novem­ber 1947)—but he was quot­ing an unknown pre­de­ces­sor. From Churchill by Him­self, page 574:
 
Many forms of Gov­ern­ment have been tried, and will be tried in this world of sin and woe. No one pre­tends that democ­racy is per­fect or all-wise. Indeed it has been said that democ­racy is the worst form of Gov­ern­ment except for all those other forms that have been tried from time to time.…
 
So, although these are Churchill’s words, he clearly did not orig­i­nate the famous remark about democ­racy.