Varen the Unbelievable!!!
But seriously, everyone knows its Godzilla. When the cloverfield monster or whatever can shoot atomic fire beams, you let me know.
Note: only the traditional man-in-suit Godzilla qualifies - the CGI Godzilla from that awful Hollywood movie just doesn't deserve to be called Godzilla.
I don't see how anybody can even try to argue anybody but Godzilla as the greatest movie monster of all time. There are lots of movie monsters, but only one Godzilla, who will reign forever supreme, because he is Godzilla.