Comparing https://m.imdb.com/title/tt0052077/reviews (“Plan 9 from outer space? reviews) with https://www.imdb.com/title/tt0060666/reviews (“Manos: the hands of death” reviews), I don’t think it qualifies as “worst movie ever”.
One of the reviewers of ‘Manos’ says: “What can I say about a movie so bad that it makes Plan 9 From Outer Space look like Casablanca?”, another “I have endured a thing or two in my life: Plan 9 from outer space. Hercules against the moon men. Godzilla versus Megalon. German musical comedies from the early sixties. However, THIS was too much for me. About one hour or so into the movie, I quit.”
I have watched Manos.
I am very glad I had alcohol to go with it.
Maybe the "worst" but at least it's not boring like some more recent big-budget ones are. (Incidentally, there's some enjoyment to be had watching B-rated movies from the late 50s to the 80s.)
"Plan 9" is probably close to the worst film you'd watch for fun.
"Manos" is worse to the extent slogging through it is a chore, at least without commentary to add some entertainment value.
It's of course not relevant in the current context but still fun so I dare add that it is famous because it's so bad. It's known as "worst movie ever". It is worth watching the start and a few scenes - even if one does not have the patience for the whole thing - just for some laughs. It already starts (4 minutes in) with a scene in a hilarious airplane cockpit, which does not look like one at all (those controls..!). The zombies are really funny too.
Full movie: https://youtu.be/Ln7WF78PolA