tohellwithyourcrap said:
Firefly was his origin, without it, we get no Avengers, or any of his future work that we'll still be talking about for years to come.
Actually, Buffy the Vampire Slayer was his origin. Firefly aired somewhere near the end of its run. And while he had done several things before Buffy (including co-writing on Toy Story!), that's really what put him on the map. In fact, without Buffy, Fox would've never agreed to Firefly in the first place.
THE MORE YOU KNOW
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In all honesty, I think that a fulfilled life could be lived without seeing either of those. As much as I love Firefly, it's not for everyone, and in the end it just leaves you wanting more (if you did like it).
My nomination would go to The Simpsons, until about season 7 to 10. Not that I'm one of those who thinks that's where it stopped being funny, oh no, I still love the Simpsons. Rather, the early Simpsons stuff is what shaped modern pop culture to what it is today. I say the "necessity" stops between seasons 7 and 10 because at that point, Simpsons started being influenced by itself more than pop culture (probably due, in part, to how much it had affected pop culture), and as a result the cultural impact it had in the first place weakened significantly.