WetGeek
I thought about adding the classic "mad doctor films," such as Frankenstein, The Invisible Man and Doctor Jekyll and Mr. Hyde, but these are typically considered to be horror films. I suppose they could go either way, because The Fly and X: The Man With X-Ray Eves are also essentially mad doctor films. There aren't any mad doctors (the science element of sci fi) involved in The Mummy, The Wolf Man, Dracula and (a personal favorite growing up) The Creature from the Black Lagoon. Where do you draw the line?
Any way, special effects aside, they don't make 'em like they used to, eh?
EDIT: Oh, and how about King Kong and the original Japanese version of Godzilla? Do these count as sci fi? They are classics, none the less!