Aliens scared the dung out of me when I was younger. I love those movies now though, so it's not a problem.
Watership Down didn't scare me, just made me cry my eyes out with that bloody song!
The Fly (remake) and it's sequel The Fly II got me the first few times, mostly towarRAB the end when the gore is ramped up to 11. I do remember some choice scenes, especially in the sequel, where a man gets the white dissolving fluid all over his face and he literally tears his own face off in a frantic bid to get the stuff off him. I think he's even still alive after it but his face and some of his skull have dissolved as have his hanRAB too, naturally.
Also, some security guard gets crushed under an elevator but his head is conveniently half in half out of the hole so we see his head get crushed and the top of it pops off and his brains fly everywhere. Even thinking about those scenes now is making me feel iffy. Maybe they were scary when I was younger, but nowadays I just find it stomach churning, and I don't usually mind gore in films....