Here is a list of my all time favorite movie monsters, in no particular order. All of these monsters have been talked about to death, so I'll merely provide a brief comment about them.
The Predator (Predator - 1987)
Although I said no particular order this character may take the #1 spot. Stan Winston's creature design is absolutely amazing. The film and the story behind the Predator has captured my imagination since the first time I saw the movie.
Godzilla (Godzilla - 1954)
King of the Monsters... that pretty much says it all.
The Alien (Alien - 1979)
In world of sci-fi and horror few can deny that HR-Gieger's nightmarish design of the titular creature is breathtaking. It's a monster that has stood the test of time. Various design changes have occurred since the first film, but the structure of the original design is there and stands up to this day.
Jaws (Jaws - 1975)
This movie monster single handedly put the fear or sharks into the public consciousness, which has remained to this day.
Freddy Krueger (A Nightmare On Elm Street - 1984)
Although, many consider him a "slasher" type of villain, I see him as a monster because of of his powers and way he attacks his victims. He's well beyond a Leatherface or Michael Meyers type of character.
The Creeper (Jeepers Creepers - 2001)
This creature is probably the only new movie monster in the last decade that seems to capture everything I like about an original movie monster. Mysterious, creepy and above all memorable.
The Thing (The Thing - 1982)
This is one of my favorite movies of all time. The monster in this movie, is not one particular creature or design, but that is what makes the Thing so cool, is it's ability to hide in plain sight. However, when attacked we the viewer are able to see some of the strangest creatures seen on film. The dog kennel scene is a favorite of mine.
Werewolf (The Howling - 1980)
I've always been a fan of werewolves. The transformation scene and final design of the werewolves in "The Howling" is my favorite to this day. No film other than "An American Werewolf in London" has come close to this movie magic and the movie is almost 30 yrs old.
T-800 (The Terminator - 1984)
I don't know if everyone would consider a robot a monster, but when it comes to the Terminator he's a monster in my book, not organic, but monstrous machine. Schwarzenegger's performance is great, and the endoskeleton version at the end of the film is even scarier. This is another one of Stan Winstons greatest designs.
The Gill Man (Creature from the Black Lagoon - 1954)
There were some great "Universal" classic monsters that everyone knows, but the coolest looking to me was the Gill Man. I look at the design of the creature now and it looks just as great to me as when I first saw him swim out of my TV. My first viewing was the 3D version at home.