Hhhm, I would have to say my top 5 great movie monsters are:
#1.) The Alien from "Alien" - and all of it's spawnings - the Alien Queen, the Alien Runner, the Face Huggers, the Chest Bursters, the Predalien from AvP: Requiem (especially when it's around pregnant women, lol), etc. The Alien just wins, hands down.
#2.) The Predator, from the movie of the same name.
#3.) Unicron, from the animated "The Transformers" movie.
#4.) The green orb from "Heavy Metal".
#5.) The Lord of Darkness, from the movie "Legend" - Tim Curry ftw.
And the runner up is:
OMFG... the "Fluke Monster" from that one old ass episode of "The X-Files". I know it's not from a movie, but... damn... sometimes I still get creeped out and nervous when I go to take a shit because of that episode!
Honorable Mentions:
#1.) The Rancor, from "Return of the Jedi"
#2.) The Balrog from "The Fellowship of the Ring".
#3.) The Stay-Puft Marshmallow Man from "Ghostbusters".

-Itharus