There is no such thing as 'the worst movie of all time' - there will always be something... worse!
Some of less than fond memory....
Kingdom of the Spiders
Creature of Crater Lake
Plan 9 From Outer Space (now viewable on Google)
They Saved Hitler's Brain
The Flesh Eaters
The Day the Earth Froze
A Polish Vampire in Burbank (worse than it sounds... and it sounds pretty bad!)
Mission to Mars
From the Earth to the Moon (hear that humming? That is Jules Verne spinning in his grave)
Deathstalker (any of them... I think there were 5)
Sword & the Sorcerer
Subspecies (lots of these too)
Dollman (in one scene he is a classic GI Joe figure, complete with Kung Fu grip...)
The Demonic Toys
the Demonic Toys! (It just doesn't get any better than this)
Puma Man (not fair to include this really, it's Italian... and badly dubbed.)
The Deadly Mantis (look for the word Tonka on the bottom of the school bus as the Mantis hurls it aside.)
And I know that there is worse out there, and this is coming from somebody who actually liked the House movies.
The Auld Grump