9. The Denevan Parasites "Operation Annihilate!" TOS
The story behind this episode was actually quite good: it was a fantastic idea, written by Steven Carabatsos, involving single-celled creatures that infested a population to force them into expanding and moving outward to allow the creatures to spread throughout the universe. Unfortunately, the creatures looked like flying fake vomit-piles, or large, gruesome ravioli. Clearly even a kid could work out that the best defence against these things would be to simply duck, or stay out from under ledges and shadowy areas, like you would if ever someone threw a Frisbee at you. It was basically the same principle, except that the uneven and bumpy surface of the parasites would make them less aerodynamic and slow their approach somewhat, giving you a bit of an advantage. Here's a tip if you're making a sci-fi show: if you want to threaten someone with an alien, don't imagine it to look like flying vomit.