"Welcome to the Hellmouth" (Season 1, Ep. 1)
"That gym was full of vampi… asbestos."
The premiere episode opens with a couple breaking into a high school. The boy is lascivious, even predatory, the girl nervous and scared. But then, suddenly, her face changes into something monstrous, and she bites him, draining him of his blood.
In that moment, Buffy the Vampire Slayer defined itself as a show that would subvert the expectations of genre fiction and the gender tropes that had long defined it. "Welcome to the Hellmouth" introduces us to Giles, Xander, Willow, and, of course, Buffy Summers, the character who would inspire generations as a female icon and a hero. The episode itself still manages to be fun, humorous, even a bit scary, but the fact that it set up all of what followed is what makes it great.