Haven't read it here yet and maybe it is considered sci-fi but still

IT, the terror from beyond space
There's just something irresistable about a monstermovie with IT as the title.
From hell it came is another great title but the movie itself, although I liked it a lot, is for fans only.
The classic THEM is also very good!

Classic horror is often an acquired taste though and the only option is watching them and not giving up to easily, you never know wich one you're going to like. But once you get a taste for it there's a lot to be discovered.