Hidden gems depends on your amount of exposure to the genre. One people don't always know about it The Abyss (extended cut).
Some newer robot movies that not everybody has seen: Robot & Frank (great!) and Chappie (South African, really wild).
Another new one I absolutely love: District 9.
Others I'd recommend: 12 Monkeys, AI (Artificial Intelligence), The Andromeda Strain, Slaughterhouse Five, the 1966 version of Fahrenheit 451, GATTACA, Total Recall, Metropolis, Super 8, The X Files Movies, 1984, THX 1138, Invasion of the Body Snatchers (1978), Children of Men, 28 Days Later, the older Blade Runner (1982), Brazil, Solaris (both versions), Close Encounters of the Third Kind, Soilent Green, The Giver, 2001: A Space Odyssey (of course), the old Planet of the Apes movies from the early 70's, Omega Man, Her, and Contact.
I'm not sure it's a big surprise but a lot of people like the X-Files episodes, The Lone Gunmen episodes, the Firefly episodes (and the movie Serenity that concludes the show). Netflix has had Twilight Zone on for nearly 10 years and it's always good. I think Eerie, Indiana is on Amazon Prime right now (good for pre-teens to adults).