Such is the stuff that the panels at cons are made of. Let's see how far I can get arguing it by myself (with a little help from the Internet):
* The obvious answer is Star Wars. It's over 30 years old, and still has massive appeal despite being dated in terms of the special effects. It's probably the most common introduction to science fiction – well, that or Star Trek, but that's a different panel. [audience laughter] The story is timeless, and the score is by John Williams. You can't beat that.
* I concede John Williams. But Star Wars isn't science fiction just because it has a science fiction setting. The story would work just as well in other settings. It's epic myth, in the same tradition as Homer and Virgil and Gilgamesh. Orphan boy from obscure backwater is the Chosen One with a special power to save the world from absolute tyrannical evil. That's the plot to how many stories?
* Oh, come on, you eliminate most of the genre with that argument. The ones that actually hinge on the science fiction elements are few and far between, and a lot of them just aren't good films. I mean, take The Matrix – you could practically summarize it the same way, except that the science fiction elements really are essential to the plot. That doesn't make it a good science fiction movie. Good action movie, maybe...
* What about the ones that started the genre? Just as with books, you've got to go back to Jules Verne. 20,000 Leagues Under the Sea was filmed in 1916, and I believe there was a French version even earlier (1907). A lot of the early films were science fiction.
* But have you even seen any of those? No, I didn't think so. Let's stick to films that are recognizable today. I would say the oldest of those, the first of the modern era of science fiction movies, is 2001: A Space Odyssey. It's frequently on lists of best movies of all time, so I think it would be hard to discredit it as best science fiction movie. And, okay, it's not John Williams, but the music is pretty phenomenal.
* Okay, let's open it up to questions and comments from the audience. You in the green shirt. No, the third row.
Q: [something obscure that none of the panelists know anything about, but they all fake it and move on. Or, worse, one does know, and engages in debate for five minutes boring the audience and the panel until the moderator cuts them off.]
Q: I came in late, so I missed the beginning, but isn't anybody going to mention Star Wars?
[entire audience groans]
Q: Planet of the Apes is just as old as 2001: A Space Odyssey, you know. They were both released in 1968.
A: So do you think Planet of the Apes was a better movie?
Q: Well... no.
* We're just about out of time, so let's go around one more time to wrap things up. What's your final vote?
At which point there are two votes for Star Wars, one for 2001: A Space Odyssey, and two for other movies that weren't mentioned at all during the discussion or only mentioned briefly during a digression that was mostly off-topic.
Obviously I'm summarizing the actual arguments. Expand them in full, add in five minutes for starting late, and twenty minutes or so of introductions, interjections, and digressions, and I think we've pretty much got a full hour-long panel right there, yes?
But for the record, all that notwithstanding, it's got to be Star Wars for me. It's the one I keep coming back to, the one that feels like home. You don't get better than that.