1. 2001: A Space Odyssey (1968)
    ...in which the aliens themselves are not shown. They will share none of our DNA sequences, and will not likely use DNA at all, so it may be futile to even imagine what they would look like. But to bear witness to their formidable intelligence is humbling.
  2. Contact (1997)
    ...in which the aliens themselves are not shown. But must respect their power to communicate via memories of that which is familiar to us.
  3. Invasion of the Body Snatchers (1956)
    ...in which the aliens themselves are not shown. Of course these aliens occupied our bodies, which may be more terrifying than simply laying eyes upon them.
  4. The Blob (1958)
    An alien with no DNA in common with life on Earth should look more different from life on Earth than any two life forms on Earth look from one another. Behold, The Blob.
  5. The Day the Earth Stood Still (2008)
    Once again, you don't see the alien - they occupy human form. But the birthing sequence and those pesky self-replicating hungry locusts are genius.
  6. War of the Worlds (2005)
    What the hell are those things? what were they doing to our civilization? and why were they flaying us? Just the kinds of questions that should preoccupy us in a first encounter with aliens.
  7. Independence Day (1996)
    Inspired by War of the Worlds, for this film I always wanted to do what Will Smith does -- punch an evil alien in the face and say "Welcome to Earth." These creatures look like nothing on Earth and they wielded awesome technology.
  8. Alien (1979)
    Gotta love ugly aliens with teeth. Again, terrifying -- not because they looked like terrifying animals on Earth, but because they looked like nothing we've ever seen on Earth.
  9. Star Wars: Episode IV (1977) & Episode VII (2015)
    If your aliens are going to walk & talk and have heads & arms and can play musical instruments, then the zoo of creatures in these two films wins that contest of creativity. Spielberg's ET, Star Trek's The Gorn, and The Creature From the Black Lagoon, would each go unnoticed in these crowds.
  10. Predator (1987)
    Anything that hunts humans better than the way humans hunt big game is singularly terrifying. Although still an actor in a rubber suit, its tactics, methods, and tools were entirely alien to us. And from this film we received the wisdom of the ages: "If it bleeds, we can kill it." And we not doubt all agree with Arnold's assessment that the alien was "One ugly motherfucker!",
  11. Honorable Mention: The Andromeda Strain (1971)
    Perhaps this one should be in the top ten, but then I'd have to bump one that's already there. Microscopic crystalline life form with no organic molecules. Can't get more alien than that.