Very good choices all, but may I put forward, in no particular order,
another ten fearsome faves (each with a reason why)?

****The ants in THEM (Actually, it's not their rather spindly,
windblown look which is so scary--but that eerie chittering sound
they make. I vote for the scene where the lone cop hears that sound
and walks out of the frame to his unseen, but NOT unheard demise, as
one as effectively chilling as any I can think of.)

****The title beastie in CURSE OF THE DEMON (Yeah, we see too much
of this critter too early in this most intelligent of monster movies,
but it really is a splendid and floridly hideous creation, not
something you'd want to encounter in the woods on a dark night...)

****The lively murderous brains in FIEND WITHOUT A FACE (These stop-
motion beauties are both endearing AND horrifying: a neat trick!)

****the Ymir in 20,000,000 MILES TO EARTH
****the snake woman in THE 7TH VOYAGE OF SINBAD
****Medusa in CLASH OF THE TITANS
(These are, respectively, the most exciting, the most unsettling,
and the most spine-chilling of Master Ray Harryhausen's many
monsters.)

****the tree monster in FROM HELL IT CAME (OK, the movie is a laff
riot from start to finish, but, when I was eight years old--and saw
it for the first time--for the next few weeks I uneasily went to bed
convinced that the overgrown stump with the scowling puss and the
dagger stuck in its, uh, chest was staring at me out of the recesses
of my closet just waiting for me to fall asleep...)

****the silicates in ISLAND OF TERROR (I agree that their look is
humdrum, but don't they make the most marvelously creepy sounds,
especially when they're sucking the bones out of their victims?)

****the title creature (and pals) in THE CRAWLING EYE (For sheer
over-the-top disgustingness, it's hard to beat these tentacled,
bulbous-brained ambulatory eyeballs--especially when they're busy
harvesting heads!

****the sandworms in TREMORS (I feel the earth...move...under my
feet.... I adore the elegant simplicity of both these nasty boogers
and the movie they dominate.)

****the Id monster in FORBIDDEN PLANET (Mostly heard--pinging and
whooping--and suggested--BIG footprints!--but not seen--except as
a horrific outline in a force field. Literally the stuff of nightmares
in the film--and out!)

There. That should add some bubble to our opinionated brew.
(Hey, do they add up to eleven? D'oh! Oh, well...)