Yknow, it pains me to say this. PAINS. I am not a Star Wars fan. Fact, I loath that it has turned sci-fi into a mockery through its most hardcore and visible fans. It's hard to take sci-fi seriously when you see some 400 pound greasy sweaty guy dressing up cosplay as Boba Fett and slurring his words through a lisp and crustied acne.
but Darth Vader.
When you think Sci-Fi villain, that is the first name that comes to mind. When you peel back the geekery and mostly horrid acting in the prequels - Darth was the original baddy. He even had a great story that evoked genuine empathy from the crowd in the end. That a dude who is built to epitomize evil for two and half movies, could turn around and get you to care about him, is quite rare.
On a less serious note, I'd vote Christmas Critters as a close second. They might be on a cartoon kiddy show, but they are definitely memorable, scary, evil, and hilarious. Imaginationland made them better known, but the original christmas episode made them legendary.
Squirrely: Hey look it's Santa Claus!
Woodland Critters: Yay!!
Racoony: Let's eat his flesh