Invasion of the Body Snatchers, the novel The Puppet Masters, Operation: Annihilate! from Star Trek TOS, there was even an episode of the TNG where DATA, O'Brian and Troi were taken over by entities. All non human takeovers and all of them gave me the Heebee jeebees! Non American may not understand the last word but I have a feeling they know what I mean.

Brrrrrr!