First off, I enjoy a lot of older movies in spite of their dated special effects.
War Of The Worlds (I know you can see the strings, but I still felt the spaceships were truly menacing)
Tarantula (probably the scariest of the Giant Animal films)
Any Ray Harryhausen creature (their herky-jerky movements somehow made them more frightening than any CGI monster)
However, there are some old movie monsters that are so ridiculous that it would be impossible to take them seriously.
Some of my favorites:
It Conquered The World (1956)
Night Of The Blood Beast (1958)
Attack Of The Eye Creatures (1965)
and my favorite The Creeping Terror (1964) (pictured)
What are your favorite really cheesy old monsters?