I am a SciFi nut, so this is not too easy to answer. Terminator, Blade Runner, The Island, Jurassic Park, Sphere, Contact, Logan's Run, Outland, and The
Day The Earth Stood Still (the original, not Keanu Reeves horrid remake) all garner my attention, but the first SciFi movie I remember seeing and loving was Michael Crichton's "The Andromeda Strain" years and years ago. That was and remains my favorite. The thought that a virus from outer space could come down here and kill us all was mind blowing to a young boy.