Metropolis (1927) is a great sci-fi movie if you can find it. It's for older people, though.
Metropolis is responsible for inspiring most of the futuristic city sci-fi movies.
I didn't really like the Fifth Element, it didn't even try to take itself seriously.
It's not bad, but more like a sci-fi parody.
Cube (very gory, but compelling story)
Ghost in the Shell (If you didn't like all the philosophy babble in the second and third Matrix
movies, you may not like this.)
Sphere (really neat story, with a twist ending)
A Clockwork Orange (violent movie about violence, definately not for the younger set)
Back to the Future (more humourous sci-fi, time paradoxes especially part two.)
Ray Harryhausen Films (lots of stop-motion monsters):
Mighty Joe Young: (1930's version) ripoff of King Kong, but still very good)
Earth vs. The Flying Saucers: classic, flying saucers attack Washington D.C.
20 Million Miles to Earth: Monster from a space flight to Venus gets loose in Italy, starts as the size of a cat and grows throughout the film.
Mysterious Island: Has almost nothing to do with the novel it is based on, giant animals (bees, crabs) attack people stranded on an island.
Jason and the Argonauts: Many memorable scenes, including sword fighting skeletons. Honor Blackman (Pussy Galore) plays Hera.

This was before there was computer graphics. And it still looks more realistic than half of Die Another Day.
Clash of the Titans: More greek myths, like Argonauts. Pegasus, Medusa and other
creatures.
Some of these aren't sci-fi, but neat special effects movies nonetheless.