Following his rules, I am limiting myself to only monsters appearing in films which I have seen. Thus, the Twilight critters are safe.
TOP TEN WORST MOVIE MONSTERS
#10: The Blob (1958)
For some reason, this movie terrified my mother-in-law when she first saw it. But really? It's Jell-o with an attitude. At least the theater is air conditioned. And showing a Bela Lugosi film.
#9: Creature from the Black Lagoon
Fish guy from prehistoric times? All he wants is to make it with a mammal, raise a few baby fishmen. If only they had used the Creature as the prototype for some kickass Deep Ones in a version of Shadows over Innsmouth. Instead, it's a rubber suit monster with a SCUBA gear humpback.
No, this is not a joke. In The Howling III, the action takes place in Australia. Therefore, we must have werewolves with pouches! Are these really any scarier than real werewolves? Not in the slightest. And dressing like Sister Mary Catherine doesn't scare me, either, though considering her skill with a yardstick across the knuckles, it probably should be more terrifying.
#7: I Was a Teenage [Fill in Blank] (1957)
They were both dreadful. Michael Landon as the werewolf. Gary Conway as the Frankenstein's Monster. But at least the werewolf costume wasn't as bad as that awful Frankenstein.
Any monster list that is not specifically anti-kaiju must have kaiju. I think that is internet law #647 or something. And Mothra is the lamest kaiju of them all. First, she can be controlled by two tiny Japanese fairies. Second, although she's big, her greatest power is... poisonous spores. Unless you count the spraying silk attack of the larval form.
Not a typo. That word is bunnies. Giant, carnivorous bunnies, sure. But still... bunnies? Night of the Lepus tried to make you scared of them. They also did it the lazy way (regular animal, scale setting). Ho hum. These are sure not the killer bunny from Monty Python and the Quest for the Holy Grail. After all, these are lame, and that rabbit's dynamite!
Among the many terrible things that were in Conan the Destroyer (like casting Wilt Chamberlain and Grace Jones), this monster was pathetic. A mighty sorcerer (Thoth-Amon) chooses this for his combat against Conan? I mean, you have to break some mirrors to kill it? Gee, that's tough. Think of all the bad luck you're going to have...
#3: Giant Spider from Ator the Fighting Eagle
Back in the 80s, I loved sword and sorcery movies. Which had a lot of bad monsters in them. I think the worst was this giant spider. I swear you could see the cables which manipulated the legs in some shots.
#2: Iguana Dinosaurs
A staple of the early sci-fi movies, an iguana with a forced perspective or film overlay is the laziest monster ever. This one is from One Million Years BC. Which did, however, have some redeeming qualities as a film...
#1: Robot Monster
The Ferret hit the nail on the head with this one. However, if I had ever seen one of the Twilight movies, I would have bumped this guy for the Sparkly Vampires.