I was looking at some older books and picked up a compendium from my shelf.
The Best From Fantasy and Science Fiction - 22nd Series. (1978)
Of interest here is a piece by their film reviewer, Baird Searles,  who in his time was very highly respected. He listed his ten best SF or Fantasy films of all time.
His limits were that he only listed sound movies as he classed silents as essentially a different genre.
Why this is of interest, to me anyway, is to see what a quite respected critic picked 28 years ago and to compare it with what would be there now  - and what has been forgotten.
Searles seems to be balancing somewhat, deliberately picking films of different types for his list.
That list is, in no particular order, Frankenstein, Things to Come, The Thing, The Incredible Shrinking Man, Forbidden Planet, 2001, La Belle et la Bete, Fantasia, & Carnival of Souls.
Of a little more interest to me is his runners up. Again in no particular order: The Innocents (made from the Turn of the Screw), The Time Machine, King Kong, Dracula, Dr. Phibes Rises Again(Searles says better than its prequels), Jason and the Argonauts, Blood On Satan's Claw, Village of the Damned, & Zardoz. Three Czech films,  he Emperor's Nightingale, The Tales of Hoffman & A Midsummers' Night Dream.

Searles also could not resist doing a list of the ten worst, although he points out that there are a lot more than ten. So he lists exemplars of types of bad. Alphville (trying the TELL US SOMETHING degenerates into boring nonsense) , The Giant Claw (A cheapo stupid that is so bad that it isn't even funny) , Planet of the Apes (errant nonsense passing itself off as SF), Invasion of the Body Snatchers (one bit of inconsistent plotting ruins a beautifully built concept)  and The Jungle Book (exemplifying what the Disney treatment can do to a classic.)
Remember it came from a reviewer with the bias of writing in a print Fantasy and SF Mag. If you're interested you can read the article by signing up for a free one month trial at http://books.mx.tl/read-the-best-from-fantasy-and-science-fiction-l4472086.html