The scene where a toddler is the sole survivor of the spider's attack. Rather chilling.
The opening bit, where the dad is probably getting sliced up by the web, is also pretty effective.
Not to mention the father's corpse when they do find it.
The incredibly obscure trailer you can only find in the VHS tape 'Fantastic Dinosaurs of the Movies'. It's short, but dear god, the narrator's voice cracks to the same level as the normally silly roars of the spider. Make of that what you will...