L'invasion des soucoupes volantes (1977)
Starship Invasions (original title)

While there's no denying the poor production value and cheap special effects (this is a B movie, after all); I feel the story behind this movie has some value. The motivation of the bad guys is well established. They are not simply evil, but are in a bit of a tight spot themselves. Also, the primary method they use to achieve their goal (a suicide ray) is rather chilling. The use of telepathic communication between the aliens was a nice touch. The cheesiness of this film can easily be embraced as enjoyable camp. It's not a great film, but I think you'll find that it sticks with you after you watch it. The story is fairly rich, with lots of details and connections filled in along the way. It truly has some meat to it. I first saw it as a kid while growing up. Now, over 25 years later, I finally tracked it down and watched it again. I'm still charmed by it.