I used to think the worst movie of all time was "The Charge of the Light Brigade," in which Errol Flynn engineers one of the worst disasters in British military history to avenge a Sepoy Mutiny massacre on an evil raja who for some reason is hanging out in Crimea during the war.
I am now watching the ending and apparently I forgot that 1) the Charge was a win for the Great Britain (good show, jolly good show) 2) Russian cavalry were involved and 3) Ukraine looks remarkably like California.
I also could have put money that this thing was in technicolor. B/W is more tolerable than my eyethrobbing memories.
Anyway the actually worst I've ever seen was something called Carthage in Flames which was mindnumbingly dull yet features unconvincing romance AND unconvincing sword fights. I passed out during the movie. Oooh look it's on YouTube for free! https://youtu.be/UM5EDpftGsw