the T-1000. Stealthy's posting of the hospital escape scene in T2 put the shits up me a bit. That bit where he's running at the closing lift doors. brr. Some stuff you just see at a certain point in your life, and there; it's burned in as a fear response.

See also:
the alien queen
Bob from Twin Peaks
ED-209, until someone bought me an ace toy of it, and it became my favourite thing ever.
and weirdly, Montgomery Moose from out of being in the Getalong Gang. hmm.