The regenerators were probably the most scariest thing on RE4. Especially when you shoot their legs off - you think they're dead, and then they jump on you with their massive teeth blazing.
It can't be that we're used to Resident Evil that we don't find RE4 as scary, because even when I am replaying the original games, I can easily be jumped or scared as I forget what's coming next.

For some reason - everytime I got through the automated door that leads to the kitchen in the train on Resident Evil Zero, I jump at the sound of the door. Not supposed to be remotely scary - but that door scares the skin outa me. It must be the fact that I forget it is an automatic door each time I enter it.