Well this will be hard since there have been english monarchs for like 1000 years?

Least favorite US presidents the ones that were against abolition of slavery.

Woodrow Wilson who supported the Ku Klux Klan and Herbert Hoover.

Who was that President that turned White House like a zoo??

Crap! I missed the U.S. Presidents one... I would've said William Henry Harrison: If I can't trust you to use your common sense to keep yourself alive, how can I trust you with a country???
(Honorable mentions: James Buchanan & Franklin Pierce)

Now for English monarchs, I have to say King John. So bad no other king has ever taken that name...

Oliver Cromwell easily, or should I say "Lord Protector"?