The Movie: 'Alien' (1979)
The Scene: Poor Kane (John Hurt) has had a rough day. Until very recently he's had a horrific spider-lookin' alien parasite stuck to his face -- but things are looking up. Kane's up and around, joking with his shipmates before bedding down for a long trip back to Earth ... and then the coughing starts. At first it's no big deal, but then the poor man is overwhelmed by shocking convulsions, his chest starts to buckle outwards, and then we realize what that horrific spider-lookin' alien parasite was doing on Kane's face: it was laying an egg.
And the terror hasn't even started yet.
Why It's Iconic: How many reasons do you need? It's shocking and unique. It's tragic and fascinating. It's gory and horrifying. It's a virtually flawless combination of stellar special effects and fantastic acting performances. Viscerally, the sequence packs a massive wallop, but what's coolest is how the scene of sudden horror works in a narrative sense: just as things are looking up, Act I draws to a close with one serious kick in the ass. I just love this scene.
Imitators/Flatterers: More than I could count, most likely, although the only person to recapture the dark magic of the chestburster's shocking arrival was James Cameron. Still, it'd be silly to not include the following clip from 'Spaceballs', which works mainly because Mel Brooks hired John Hurt himself to pull off the parody.