Oh god those things are awful. On exploded my house and because of it i fell into the lava pool on the first floor and died! I lost my diamond pick axe...
Chrysalids/Tentaculats are scarier than any of X-Com's bosses.
Other favorite is the Silent Hill Pyramid Head.
Disclaimer: Any sarcasm in my posts will not be mentioned as that would ruin the purpose. It is assumed that the reader is intelligent enough to tell the difference between what is sarcasm and what is not.